I use the file system tab in the settings to put check marks at the directories I want to have backed up.
E.g. I have put a check mark at the top level directory of my user account (C:\Users\Userxyz) to have all files related to me included into the backup. But when I expand the directory tree by clicking the + signs I noticed that certain directories do not appear (and hence not being backed up), although I can find those directories using the file explorer.
Only two directories contained in ...\Userxyz\AppData\Local show up, about 30 others do not. Especially the Mozilla subdirectory I would need to be included in the backup, because it contains my user profile with all settings. Sometimes Firefox forgets everything after a crash, and I do not like to recreate 50+ open tabs manually.
Any ideas? What kind of blacklists do exist preventing files being backed up?