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Firefox does not offer a standard Settings checkbox named “Tab Sleep.” Its equivalent is officially called Tab Unloading: when available system memory becomes low, Firefox can unload background tabs to reduce memory pressure and lower the chance of a crash. The tab stays in your tab strip and is restored when you select it.
To control the feature, open about:config, search for browser.tabs.unloadOnLowMemory, and set the preference to true to allow automatic unloading or false to turn it off. The change itself is the documented action; Mozilla does not require a restart. Because about:config contains advanced settings, change only the named preference and read Firefox’s warning first.
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“Tab Sleep” is a convenient description, not the label Firefox uses in its interface or technical documentation. Mozilla calls the feature Tab Unloading. It is designed for memory-pressure situations, rather than for putting every tab to sleep after a fixed period of inactivity.
When Firefox decides that available system memory is becoming dangerously low, it selects background tabs for unloading. The tab remains visible in the tab bar, but its active browser instance is discarded and the tab becomes lazy. Selecting that tab causes Firefox to recreate the browser and load it again.
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This is different from closing a tab. Closing removes the tab from the tab strip; unloading preserves the tab entry so you can return to it. It is also different from a power-saving timer: Mozilla describes a low-memory trigger, not a user-configurable number of idle minutes.
How does Firefox decide which tabs to unload?
Firefox uses a memory-pressure detector together with a tab unloader. In general, it favors tabs that have been unused for the longest time, but the priority system avoids or deprioritizes tabs whose interruption would be especially disruptive.
- Foreground tabs: the tab you are actively viewing is protected from ordinary background unloading.
- Media playback: tabs using active media are protected or given a lower unloading priority.
- Picture-in-Picture: a tab supporting a Picture-in-Picture window is protected or deprioritized while that activity is in use.
- WebRTC: tabs involved in WebRTC communication, such as an active call, are treated as important.
- Pinned tabs: pinned tabs are less likely to be selected than ordinary background tabs.
- Least-recently-used tabs: when Firefox must reclaim memory, older inactive tabs are generally considered first.
These rules are prioritization safeguards, not guarantees that a particular tab can never be unloaded. The exact decision depends on current memory pressure and the state of the browser.
How do I enable automatic tab unloading?
Enable the feature by setting Firefox’s controlling Boolean preference to true. Follow these steps in Firefox desktop:
- Click the address bar, type
about:config, and press Enter or Return. - Read the warning shown by Firefox. Configuration Editor changes are intended for experienced users and can affect stability, security, and performance.
- Use the search field to find
browser.tabs.unloadOnLowMemory. - Click the value control beside that preference so its value is
true. If the preference is not already listed, use the Boolean option to create it, then set it totrue. - Close the Configuration Editor tab. The preference change is the relevant action; Mozilla’s documented procedure does not call for restarting Firefox.
With the value set to true, Firefox is allowed to unload eligible background tabs when the browser detects low available system memory. It does not force an immediate unload simply because you changed the value.
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How do I stop Firefox from unloading inactive tabs?
Set the same preference to false. This disables Firefox’s automatic low-memory Tab Unloading behavior while leaving ordinary tab management available.
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about:configin the Firefox address bar and accept the warning. - Search for
browser.tabs.unloadOnLowMemory. - Toggle the value to
false. - Close the Configuration Editor. No restart is specified as necessary for this change.
Disabling automatic unloading does not prevent Firefox from using memory-saving techniques unrelated to this preference, and it does not remove manual unload commands if they are available in your version. It specifically tells Firefox not to perform this automatic response to low-memory conditions.
Where can I see which tabs Firefox would unload?
Firefox desktop includes a diagnostic page at about:unloads. Enter that address in a new tab to inspect the browser’s current tab-unloading information.
The page lists each tab’s unload priority, last-access information, and memory use. It also provides an Unload control for manually unloading the least-recently-used tab. This lets you reclaim memory selectively instead of waiting for automatic memory pressure.
Mozilla also documents unloading selected tabs from a tab’s context menu. Right-click a tab and choose the available Unload tab command when your Firefox build exposes it. Manual unloading is separate from the global automatic preference: you can keep automatic unloading disabled and still use a one-off unload action when you need to reduce memory use.
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What is the difference between the two Firefox unload preferences?
The names are similar, but they control different things. Confusing them is a common reason a setting appears not to work.
| Preference | What it controls | Values and effect |
|---|---|---|
browser.tabs.unloadOnLowMemory |
Automatic Tab Unloading when Firefox detects low available system memory | true allows automatic unloading; false disables it |
browser.tabs.unloadTabInContextMenu |
Whether an Unload tab command appears in the tab context menu | Controls the menu command’s availability; it does not switch automatic low-memory unloading on or off |
If your goal is to stop Firefox from unloading tabs on its own, change browser.tabs.unloadOnLowMemory, not the context-menu preference. If your goal is to perform manual unloads from a right-click menu, the second preference determines whether that command is offered.
Should I use automatic unloading or unload tabs manually?
The right choice depends on whether you prefer an automatic safety valve or precise control over which tabs are interrupted.
| Choice | Best for | Trade-off |
|---|---|---|
| Automatic unloading enabled | Large tab collections, systems that regularly approach their memory limit, and users who want Firefox to react before memory pressure causes instability | Firefox chooses candidates according to priority rules, so a background tab may need to reload when selected |
| Automatic unloading disabled | Workflows that prioritize keeping every tab’s browser instance available and users who prefer to manage memory themselves | Firefox will not use this particular low-memory safeguard; severe memory pressure can still affect responsiveness or stability |
Manual unloading through about:unloads or the tab menu |
One-time cleanup, troubleshooting, or a deliberate choice to unload specific inactive work | You must monitor memory and repeat the action yourself |
The preference is global for the Firefox profile. Mozilla’s documented controls do not provide a per-site exception list for automatic unloading, so you cannot use this setting to allow unloading on some domains while permanently protecting others.
Why are my tabs reloading when memory is low?
A reload can be the visible result of Tab Unloading. When memory pressure rises and automatic unloading is enabled, Firefox may discard the browser instance behind an eligible background tab. The tab’s entry remains in the strip, and selecting it starts the restoration process.
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To check whether this is the cause, inspect browser.tabs.unloadOnLowMemory in about:config and review the tab priorities and memory figures in about:unloads. If the preference is true, Firefox is permitted to take this action. If you need uninterrupted browser instances and accept the memory risk, set it to false; if you need to free memory, leave it enabled or use a manual unload.
Remember that the feature is intended for low-memory conditions, not ordinary tab switching. A tab that has been untouched for a short time is not automatically unloaded merely because it is inactive. The decision is tied to available system memory and Firefox’s prioritization logic.
Why can’t I find a Tab Sleep checkbox in Settings?
Firefox’s normal tab controls are in Settings under the Tabs and browsing area, but the automatic unloading switch is an advanced preference rather than a standard checkbox. Mozilla’s June 16, 2026 settings redesign reorganized options while preserving existing preferences; it did not move browser.tabs.unloadOnLowMemory into the ordinary tab-settings page.
That is why searching Settings for “sleeping tabs” or “tab sleep” may produce no matching control. Use the exact preference name in Configuration Editor instead. If you only want to unload a tab occasionally, use about:unloads or the context-menu command rather than changing a global preference.
What are the risks of changing this setting?
Mozilla warns that Configuration Editor changes can affect Firefox’s stability, security, and performance. The safest approach is to change only the named Boolean, avoid editing unrelated preferences, and record the original value so you can restore it.
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- Use
truewhen automatic memory relief is more important than keeping every background tab continuously instantiated. - Use
falsewhen a reload or restoration delay interferes with a workflow and your system has enough memory to handle the open tabs. - Check
about:unloadsbefore making a broad change if you are diagnosing a specific memory problem. - Keep important media, Picture-in-Picture, WebRTC, and pinned-tab behavior in mind: Firefox already gives these tabs higher protection, but protection is based on priority rather than an absolute promise.
There is no Mozilla-published user-facing percentage that guarantees how much RAM one unload will save. The result varies with the page, its scripts, the number of open tabs, and the system’s current memory state.
What should I choose?
For most users who routinely see Firefox consume available memory, leaving browser.tabs.unloadOnLowMemory at true provides a useful automatic safeguard. If preserving continuously active background pages matters more than memory headroom, set it to false and reclaim memory manually when necessary. In either case, use about:unloads to understand Firefox’s priorities instead of guessing from tab position alone.
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Can I exempt one website from Firefox Tab Unloading?
Mozilla’s documented control is browser-wide and does not include a per-site exception list. To protect a particular workflow, keep that tab active when possible or disable automatic unloading and use manual unloading for other tabs.
Does Firefox report exactly how much memory an unloaded tab saved?
Firefox’s about:unloads page shows memory-use information and unload priority, but Mozilla does not publish a guaranteed RAM-saving figure for each unload. Savings depend on the page and the system’s state when unloading occurs.
Is about:unloads available on Firefox mobile?
Mozilla documents about:unloads as a Firefox desktop diagnostic page. The documented desktop procedure does not provide an equivalent mobile interface.
What did Firefox originally intend Tab Unloading to prevent?
Mozilla’s Firefox 93 engineering explanation described the feature as a response to imminent crashes caused by memory pressure. Current behavior should be understood from the present preference and diagnostic controls, which focus on low available system memory.
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