It's possible for Firefox to forget about:config preferences you've set
Firefox has a user preferences system, exposed through its 'Settings'
or 'Preferences' system (also known as about:preferences) and also
through the more low-level configuration editor (aka about:config).
As is mentioned there and covered in somewhat more detail in
what information is in your profile,
these configuration settings (and also your preferences settings)
are stored in your profile's
You might think that once you manually set something in about:config,
your setting will be in prefs.js for all time until you go back
into about:config and change or reset it. However, there's a way
that Firefox can quietly drop your setting. If you've set something
in about:config and your setting later becomes Firefox's default,
Firefox will normally omit your manual setting from your prefs.js
at some point. For example, if you manually enable HTTP/3 by setting
true and then Firefox later makes
enabling HTTP/3 the default (as it plans to), your prefs.js will
wind up with no setting for it.
(You can guess that this is going to happen because Firefox will un-bold an about:config value that you manually change (back) to its initial default value. There's no UI in about:config for a preference that you've manually set to the same value as the default.)
For the most part this is what you want. It certainly acts to clean up old settings that are now no longer necessary so your prefs.js doesn't explode. However it can be confusing in one situation, which is if Firefox later changes its mind about the default. Going back to the HTTP/3 situation, if Mozilla decides that turning on HTTP/3 was actually a mistake and defaults it to off again, your Firefox will wind up with HTTP/3 off even though you explicitly enabled it. In some circumstances this can be confusing; you may remember that you explicitly turned HTTP/3 on, so why is it off now?
HTTP/3 is a big ticket item so you might have heard about Mozilla going back and forth, but Mozilla also changes the defaults for lots of other preferences over time. For instance, I've tweaked my media autoplay preferences repeatedly over time, and I suspect I've had Firefox updates default to some of them (removing my prefs.js settings) and then possibly change later.
If you have any settings that are really important to always be there, I think you may be able to manually create a user.js with them. Otherwise, this is mostly something to remember if you ever wind up wondering how something you remember explicitly setting has changed.
PS: To be clear, I think that Firefox is making a sensible decision (and probably the right decision) in not having a special state for 'manually set but to the same value as the default'. That would need a more complicated UI and more code for something that we almost never care about.