Is the about the profile in "AppData\Roaming" used for storing your personal data or the profile in "AppData\Local" that stored temporary data like the disk cache or the sum of both locations? Did you check what files and folders take up this space? This could be about the storage folder.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

The problem seems to be in the AppData\Roaming Firefox profile, in the folders archives (701MB) and default (821MB). I don't pretend to know what these folders contain. Also, I wasn't even aware that there is an AppData\Local directory. Roaming size is 2GB, while Local size is 1GB. I have never backed up Local as I didn't know it existed (should I?). BTW, thank you for your help.

In which folder do you see the archives and default folder? The /storage/default folder stores both website data (https+++) and extension data (moz-extension+++), so be careful with cleaning up this folder.

See also folders like saved-telemetry-pings and datareporting,

I have never backed up Local as I didn't know it existed (should I?).

No need to back up, it just has temp files like cached pages.

Thank you, and Happy New Year!