Is it a known problem that Time Machine does (sometimes) not backup Thunderbird profiles completely? I restored my system completely from a Time Machine backup only to find out that for quite a time about 5 Gigs of data are missing in the backup of my Thunderbird profile.

I can follow the weeks back where it started, but it doesn't help as those files would be weeks old and not contain the ones from, say, yesterday.

There is no folder manually excluded via Time Machine configuration. It's just missing.
Besides the Time Machine configuration there is another way to exclude files and folders from being backed up with Time Machine: the com_apple_backup_excludeItem attribute.

To find all files or folders with this attribute enter in Terminal:

mdfind "com_apple_backup_excludeItem = com.apple.backupd"


To check if the attribute is set for a single file or folder use:

xattr -x /path/to/item


If you get a result similar to this one: com.apple.metadata:com_apple_backup_excludeItem the item is excluded from a Time Machine backup.

Or:

tmutil isexcluded /path/to/item


If you get the result -    /path/to/item the item is excluded or +     /path/to/item the item isn't excluded.

To remove this attribute from an item use:

tmutil removeexclusion /path/to/item