As an experiment, I profiled for 20 minutes a Firefox instance that only had a single tab: about:blank.

Here is the result: https://share.firefox.dev/3DZp81b

Every 5 minutes there's a burst of activity in both the parent process and the Privileged Content process (very visible when looking at the memory tracks).

Bug 1735408 makes the markers on the Timer tracks very noisy, but using the console, I could get the list of timers that run with a 300,000ms delay:

• StorageActivityService
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• IdleRunnableWrapper::SetTimer
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• setTimeout() for _getTimeoutPromise/timeoutPromise</<[Sqlite.jsm]:JS
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• DataStorageTimer
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• setInterval() for init/this._recordContentProcessCountInterval<[BrowserUsageTelemetry.jsm]:JS

• IntermediatePreloadingHealer
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• setInterval() for init/this.intervalId<[SystemTickFeed.jsm]:JS
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• setInterval() for init/_intervalId<[MomentsPageHub.jsm]:JS
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• setTimeout() for _rescheduleTimeout/this._schedulerTimer<[TelemetryScheduler.jsm]:JS

The visible bumps in the memory tracks seem to match in the marker chart with the markers for setInterval() for init/this.intervalId<[SystemTickFeed.jsm]:JS (https://share.firefox.dev/2Z7scco)

Also, when looking at the memory tracks, it looks like the memory use increases every 5 minutes.

This needs to be split to several bugs. I guess this is more like a meta bug.

The meta keyword is there, the bug doesn't depend on other bugs and there is no activity for 12 months.
:fdoty, maybe it's time to close this bug?

:florian has stated he will file more specific bugs about getting this issue fixed. It has been recorded to still be happening

(In reply to Florian Quèze [:florian] from comment #0)

Bug 1735408 makes the markers on the Timer tracks very noisy, but using the console, I could get the list of timers that run with a 300,000ms delay

Bug 1817034 made the timer markers dramatically easier to understand.

• StorageActivityService

Bug 1819135.

• IdleRunnableWrapper::SetTimer
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• setTimeout() for _getTimeoutPromise/timeoutPromise</<[Sqlite.jsm]:JS

I don't remember what these two were, but I'm not seeing them in current profiles. The IdleRunnableWrapper::SetTimer one could be bug 1818881.

• DataStorageTimer

Bug 1817305.
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• setInterval() for init/this._recordContentProcessCountInterval<[BrowserUsageTelemetry.jsm]:JS

Bug 1772429.

• IntermediatePreloadingHealer

Bug 1825720.

• setInterval() for init/this.intervalId<[SystemTickFeed.jsm]:JS

Bug 1821490. Afaik, this was the one causing the most activity.

• setInterval() for init/_intervalId<[MomentsPageHub.jsm]:JS

To be filed.

• setTimeout() for _rescheduleTimeout/this._schedulerTimer<[TelemetryScheduler.jsm]:JS

The timer only fires every 5 minutes while the user is active, and switches to every hour when the user is idle. See bug 1825600.