Mozilla updates queries happen at regular intervals
This was reported to Mozilla but I thought it would probably not get considered properly unless reported here:
Fingerprintable information in update behavior
If update checks are enabled, Firefox seems to perform them at exactly the interval specified in the app.update.interval preference. (Tested with a 120-second interval and leaving the browser running.) This leads to a minor potential way of fingerprinting users on anonymizing networks like Tor because output relays can observe an update check occurring at a precise second corresponding to a particular user.
I realize this is a minor issue and difficult to exploit, but the solution is also appropriately minor. I assume it will be enough to simply randomize the scheduled time of next update (or the time stored in the lastUpdateTime settings, whichever) by up to 5% of the update interval. This fix will still preserve the user-set meaning of the app.update.interval setting, on average.