Firefox library is blank and will not fill the screen.

[Sebastian42]

My Firefox in x64Win10ProV21H1 would not open the functional window that I expect after clicking on 'Show All Bookmarks'; eventually I used Revo Uninstaller to remove it, and installed v89. Since that did not fix the problem, I used Revo again to uninstall it, and also deleted all the Firefox entries that RegScanner found in the Registry. Some 10 entries resisted deletion - most related to Avast. Another re-install improved the situation in no way. Is there a solution to this ?