There is a recent issue (Firefox 67) where a crash trashes the prefs.js file and Firefox creates a new one with default settings. That is still under investigation.

If you are quitting Firefox normally without a crash, does anything in the following article look relevant:

How to fix preferences that won't save

jscher2000 said

There is a recent issue (Firefox 67) where a crash trashes the prefs.js file and Firefox creates a new one with default settings. That is still under investigation. If you are quitting Firefox normally without a crash, does anything in the following article look relevant: How to fix preferences that won't save

Mine isn't crashing; I'm talking about closing and reopening normally.

Under the subsection of that article, "Locked preferences file",

>make sure that your user has write permissions to the file.

prefs.js has permissions rwxr-xr-x, so owner indeed has write perm though all programs don't. But the permissions, if changed (to, say, rwxrwxrwx), are also reset to rwxr-xr-x, and the file itself seems to be modified intermittently as I use Firefox. So it's being re-touched in real time, ostensibly undoing any changes every time.

I'm not brave enough to delete it outright since that page says that it'll clear all of my extensions' preferences.

You shouldn't modify files like prefs.js in the profile folder while Firefox is running. You should only do that when Firefox is closed. Firefox keeps a lot of data in memory and only writes to disk when you close Firefox. Prefs.js doesn't store extensions data, this was only the case for Legacy extensions. WebExtensions store their data currently in indexedDB (storage folder) in the profile folder,

For the record, I never modified prefs.js while Firefox was running.

I replicated the issue just now, for clarity: 1. I closed Firefox. 2. I edited prefs.js to set my services.sync.syncInterval from 600000 to 50000. 3. I saved and opened Firefox and looked up services.sync.syncInterval in about:config. 4. I saw the value at 50000. Before my eyes, in about:config, the number changed to 90000. Not long after, inbetween going from that tab to the tab of this page and back again, it changed again, back to 600000.

This behavior is pretty mystifying. Firefox modified the value TWICE as it was running.

I say I see edits to the file because I have a file explorer window open that sorts files by mod date, and prefs.js dances around with some other files as they're all being touched, which is additional evidence that prefs.js is being edited by Firefox as it runs.

Some prefs like this Sync pref are set automatically by Firefox and can't be modified. This is the case for most of the prefs that control how Sync works. Prefs that you set via the Options/Preferences page should work as expected. The services.sync.syncInterval is used internally to schedule the next Sync.

• https://dxr.mozilla.org/mozilla-release/source/services/sync/modules/policies.js#436

cor-el said

Some prefs like this Sync pref are set automatically by Firefox and can't be modified. This is the case for most of the prefs that control how Sync works. Prefs that you set via the Options/Preferences page should work as expected. The services.sync.syncInterval is used internally to schedule the next Sync.

• https://dxr.mozilla.org/mozilla-release/source/services/sync/modules/policies.js#436

Thanks. That indeed seems to be an exception; other fields like browser.display.focus_text_color are not overwritten and do survive as normal.

As an aside, since this was motivated by me wanting to increase the frequency of my history and bookmark syncs to other devices (on desktops but also particularly on a mobile device), can I do that?

I also asked whether this can be done on the mobile client here: https://support.mozilla.org/en-US/questions/1262051

firefoxsupportforumusername1 said

As an aside, since this was motivated by me wanting to increase the frequency of my history and bookmark syncs to other devices (on desktops but also particularly on a mobile device), can I do that?

The function cor-el linked to indicates that Firefox may set syncInterval to one of these preferences:

• services.sync.scheduler.activeInterval = 600 => 10 minutes
• services.sync.scheduler.idleInterval = 3600 => 60 minutes

If we assume that 600000 is in milliseconds, so it is 600 seconds, then it matches activeInterval. So maybe try setting activeInterval to 300 (5 minutes) and see what happens?

jscher2000 said

firefoxsupportforumusername1 said

As an aside, since this was motivated by me wanting to increase the frequency of my history and bookmark syncs to other devices (on desktops but also particularly on a mobile device), can I do that?

The function cor-el linked to indicates that Firefox may set syncInterval to one of these preferences:

• services.sync.scheduler.activeInterval = 600 => 10 minutes
• services.sync.scheduler.idleInterval = 3600 => 60 minutes

If we assume that 600000 is in milliseconds, so it is 600 seconds, then it matches activeInterval. So maybe try setting activeInterval to 300 (5 minutes) and see what happens?

Thanks! I set services.sync.scheduler.activeInterval to 60 (1 minute), and, although not timer-perfect and order-unpredictable, my history now syncs between two desktops about every minute.

Since syncing to/from a client is bottlenecked by the activeInterval variable on that client, and since this variable seems inaccessible on Android Firefox (it's not in its about:config), I take it this can't be done on mobile.