What are the correct settings to aggressively throttle background tabs in Firefox?

Years ago I stumbled on this post explaining various about:config settings in firefox for throttling background (and foreground) tabs, but I could never make sense of their meaning. It is unclear, for example, what the units of each of these options are (seconds, milliseconds?) and whether or not increasing the value will throttle tabs more or throttle them less.


Specifically, let's take a super-aggressive throttling policy: I want to make it so background tabs are granted only 1ms of execution time every 30 minutes. I want this policy to go into effect 1ms after the tab is no longer in the forground. That is to say, tabs should never exceed 0.00% CPU usage for a period of 30 minutes after I leave a tab in the background.

What should the values for these firefox settings be to achieve this aggressive throttling behaviour?


tabs should never exceed 0.00% CPU usage for a period of 30 minutes after I leave a tab in the background.

This is achieveable by setting the following entries in about:config

dom.min_background_timeout_value 1,800,000
dom.min_tracking_background_timeout_value 1,800,000
dom.timeout.throttling_delay 1

According to the "Inactive tabs" section of the mozilla documentation on window.setTimeout:

To reduce the load (and associated battery usage) from background tabs, timeouts are throttled to firing no more often than once per second (1,000 ms) in inactive tabs.

Firefox implements this behavior since version 5 (see bug 633421, the 1000ms constant can be tweaked through the dom.min_background_timeout_value preference). Chrome implements this behavior since version 11 (crbug.com/66078).

Firefox for Android uses a timeout value of 15 minutes for background tabs since bug 736602 in Firefox 14, and background tabs can also be unloaded entirely.

So the default value of dom.min_background_timeout_value on firefox is 15 minutes (actually set to 900,000 as the unit is ms), which makes sense for a device trying to preserve battery and scarce RAM/CPU resources. Doubling that value to achieve 30 minutes = 1,800,000.

Note that there's a distinct entry in about:config for throttling tracking scripts (dom.min_tracking_background_timeout_value) that should also be increased to the same value of 1,800,000 ms.

By default, tabs aren't throttled when they're immediately no longer in the foreground. So we set dom.timeout.throttling_delay to 1 ms to begin throttling tabs almost immediately once they're no longer in the foreground.

I don't know what most of those other about:config entries do. The budget ones are particularly confounding, and further clarification is welcomed.


For more complex workloads (letting timers run more frequently if they are short), see this mailing list post. It maintains a budget for timers, allowing them to only run up to said budget. In particular, setting dom.timeout.background_budget_regeneration_rate to a larger value will reduce the rate at which a timer is given more time to run.

How this interacts with the more traditional timeout mechanism described by Michael is unknown to me.

  • It should not be necessary to follow a link to learn the answer. Please include all of the information relevant to the solution in this post, leaving the link for proper attribution and "extra study". – music2myear 2 days ago
  • @music2myear I think all information is present: increasing the regeneration rate is sufficient to throttle background tabs. This answer is meant to be read in conjunction with the other answer, which leaves it as an open question as to what the budget options mean. I lack the reputation to leave a comment in this community, hence the separate answer. – Calum Jackson 2 hours ago

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