You’ll need to check those files individually to see if they are causing issues. Look for lines related to what’s been mentioned so far: “locale”, “firefox”, “gdk”, etc.
Or you can post their outputs here. Again though, please format it properly after you paste it into the editing text box by selecting the pasted text and pressing CTRL+E.
Or press this button with the pasted text selected.
You definitely do. Finding out what exactly can be difficult. So, to avoid nitpicking at every setting, you could instead replace the old user like this:
Copy/Backup your older user account to a folder (preferably on an external device).
Delete your old user account.
Recreate your old user account with the same exact username.
Copy only the most essential settings from your backed up folder.
Carry on like nothing happened.
To do this efficiently/effectively, it would be a good idea to temporarily install another file manager for Step 3 and onward. Uninstall it once finished.
Lastly, but importantly, before doing this, figure out what settings are important to keep, and check your user permissions so you can add them to the recreated account.