Late yesterday the Install/Update team enabled updates in the background, when Firefox is not running for browsing, for our Windows Nightly population. These changes should be making their way to a Nightly near you shortly. Please be on the lookout for update-related wonkiness and unexpected CPU and I/O spikes, and file regressions against Bug 1703909 . This functionality will ride the trains and will have a staged rollout during both the Beta 89 cycle and the Release 89 cycle.
Updates when Firefox is not running work as follows: the default Firefox profile (for each OS-level user) will schedule OS-level tasks that run periodically . These tasks invoke Firefox in a stripped-down headless “background task mode”  that pumps the update cycle before exiting. These tasks have been designed to not process updates when other Firefox instances are running, so they should not force restarts of running instances; and they access (lock) the default profile for only a very short period of time so they should not prevent starting Firefox for regular browsing. If you need to disable this functionality, about:preferences should show a checkbox in the “Updates” section for you to disable, or you can set the BackgroundAppUpdate Firefox policy to false.
If you live inside Firefox Nightly (on Windows), these changes should not impact you very much, since the tasks will start, recognize there is an existing instance, and essentially never do any work. But if you are an intermittent Firefox Nightly user, you should find yourself up-to-date a little more often.
A number of fine folks contributed to this project, including Kirk Steuber (:bytesized), Adam Gashlin (:agashlin), Rachel Tublitz (:rtublitz), Molly Howell (:mhowell), and Dave Townsend (:Mossop). The preparatory work stretches back years! As always, thanks to our amazing QA team and the many reviewers along the path. The meta ticket for this milestone is Bug 1689520 .
Questions and comments are best on Matrix in #install-update .
Nick Alexander (:nalexander), for the I/U team
 https://bugzilla.mozilla.org/show_bug.cgi?id=1703909, first appears in build ID 20210412213434.
 Right now, every 7 hours: https://searchfox.org/mozilla-central/search?q=app.update.background.interval&path=
 Meta ticket is https://bugzilla.mozilla.org/show_bug.cgi?id=1689520