(browsers in general, not specific to PM...)
For me, usability & stability are of prime importance.
These days, I do not find any browser that meets those (what should be minimal) criteria.
That said, I use what I'm comfortable with.
Then I dabble with browsers that may do certain things better then what my comfortable browser does.
(on the FF end, more specifically...)
I treat FF as a trash browser.
Don't care what I do to it, don't care what I throw at it.
If it works, it works, if it breaks, it breaks. I don't worry myself about it.
I no longer consider FF to do anything in the users interest.
I no longer expect FF to act (UI/UX) in a reasonable manner.
I no longer expect Mozilla to fix "usability" bugs.
I do not expect FF to be stable.
Given times, & FF update cycle, just about every time you turn around, a new version is out, & with that something breaks...
So to me, at this point with them, whether you are on Release or Nightly, there really isn't a whole lot of difference.
That said, at some point in time, Nightly got a fix (finally) to a bug that was really bugging me.
So at that point, I jumped to Nightly, & have just remained there (considering I don't consider FF to be particularly usable or stable, & I treat it as a trash browser).
So my reason, for (FF) unstable (Nightly) was because of a fix that came through there (now since in Release) & there just isn't any particular reason to drop back to Release.
So, every so often, I'll hit a bug, or some "feature" that bugs me to no end, & file a Bug report.
I don't particularly expect any action to be taken on any Bug report, but still do (report, sometimes).
(Never used PM unstable. Maybe I should?)
(I think I went overboard, & not exactly what you're looking for. But as no one else...
.
Guess I just should have said, oh, I try to help out by keeping an eye open for things that are wrong so that they can get fixed in a timely manner - before turning up in a release
.)