It appears that in later versions of Firefox (at least 78esr and 90.0.1... maybe earlier) that there are files that DO NOT CLEAR when the clear cache option is selected in Edit/Preferences/Security/History/Clear History. I discovered this in my re-master, and it ate up a lot of the save file, continuing to grow.
This long-winded post in AtomicPup22 addresses the issue with steps needed to mitigate, but not solve the bug.
Automating Step 8 upon Browser close would be of good service. I jusst don't know how/where to script the 'solve'.
Regards
8Geee