It is something that I have been noticing on several occasions, when a new version of Firefox is installed some of my customised setting are overwritten and revert back to some default that is different from what was there previously.
Specifically, I have noticed this with the pdf viewer. I do not use the built-in viewer as I do not like it, and most of my work with pdf's involves me having to sign the pdf's digitally with the certificate on my E-ID. Therefore I have Firefox set to open pdfs in a native application and to not use the built in viewer as that leads to too many steps to do what otherwise is one or two steps before being able to do what I want to do.
On several occasions now I noticed that when installing an update pdf files open again in the internal viewer. I know how to change it back, using the settings and even how to disable the pdf viewer in about config but it keeps getting reset. This would not be a huge problem on one machine but having to do this on all computers I regularly use, 4, every time an update is installed, is getting old very fast.
I am running Firefox on fedora linux 36, but have seen the same on windows and other linux distros. Can you please stop doing that ?