Firefox is opened with a tiny window size after upgraded to 90. What’s the problem?
Try (with firefox not running) renaming ~/.mozilla to something like ~/.mozilla.bak then restart firefox and see if there is a difference.
~/.mozilla contains the config files for firefox and maybe something you have done in the past is causing that.
Renaming the directory (as opposed to removing it) allows you to recover things like your bookmarks, etc. once you figure out exactly which setting is causing the problem.
I don’t use firefox so I cannot be more detailed in my suggestions as to which file contains the window size config.
Did you maximize the window, quit Firefox, and it still opens to a tiny window?