If you're not already using ESR, the experience of switching is horrible.
When I did this to work around a Firefox bug which broke some of my constantly-used extensions, I found:
If you switch from a newer version to an "older" major rev, Firefox deliberately discards *all* of your settings and personalization, on the grounds that the new ones *might* possibly be incompatible.
AFAICT restoring them requires figuring out the profile files and their format and editing them directly.
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@mcc
It's a very user-hostile move, isn't well-explained, and doesn't even hint at ways to work around or undo it.
Edit: And frequently, the reality is that there is no particular incompatibility. So much of the time, it's trashing settings and customizations people have built up over years, over nothing.
Very ill-considered, though nothing compared to this adtech move.