After looking around, you might be facing an issue with Flatpak permissions. Here's a comment I found on Reddit created by u/avamk where they solved the same issue.
All right, after much trying I installed Flatseal and enabled the Firefox Flatpak's access to my home folder.
After this, it created the expected profile folder under ~/.mozilla/firefox/. I put my custom userChrome.css into that profile folder, and >enabled it in my about:config under toolkit.legacyUserProfileCustomizations.stylesheets per these instructions.
With all of these, the custom userChrome.css is finally working!
As someone else has pointed out, it really depends on your use case. Although I personally keep my drives (SSD & HDD) in a redundant RAID configuration as my data is largely mission-critical.
Not OP, but I also use flat keycaps, so I thought I'd chime in. I'm not really sure why your hands would hurt with flat keycaps, but one of the reasons I chose to use flats were to allow for easier chording with Plover.
Looks nice! I daily drove something like this for a couple of years, then transitioned to a split ortholinear layout, so a bit different but pretty much the same thing.
After looking around, you might be facing an issue with Flatpak permissions. Here's a comment I found on Reddit created by u/avamk where they solved the same issue.
Link to original post: https://libreddit.tiekoetter.com/r/firefox/comments/rq40cj/cant_find_profile_folder_for_firefox_flatpak_on/