I have just installed Firefox 79.0 (release version) and can confirm the problem.
If an extension makes changes to the new "storage-sync-v2.sqlite", the files ".sqlite-shm" and ".sqlite-wal" are not inserted into the "storage-sync-v2.sqlite" when Firefox is closed. If they are deleted by the CCleaner rule "Internet Cache", all changes will be lost.
You should therefore remove the deletion of the ".sqlite-shm" and ".sqlite-wal" files from the rule "Internet Cache". It is also not necessary that these files are deleted by the rule "Internet Cache", because there is still the rule "Compact Databases".
The rule "Compact Databases" does everything right, because it merges the ".sqlite-shm" and ".sqlite-wal" files with the ".sqlite" file and compresses it afterwards.
@circcc has already written the same thing.
The only workaround is to exclude "*.sqlite-shm" and "*.sqlite-wal".
Possibly interesting: https://blog.mozilla.org/addons/2020/07/09/changes-to-storage-sync-in-firefox-79/#comment-226930