[MBAtarga]

Don't install the newest version of FF (91) if you want to preserve the current look and feel of the UI in place!

V 91 removes the ability to keep tab folders, menu items and bookmark menus as they have been previously. The "new" look adds extra gaps and spaces everywhere! Bookmarks are no longer visible on a single page - you have to scroll down a list if you've got more than just few dozen.

V 89 introduced these changes but you could undo them by altering the config settings for the "proton" design interface. With V 91 - they've removed this capability so without going through technical code changes using CSS - you are stuck!