It is recommended to backup the entire Thunderbird profile. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

I'm aware of this. That is the simple solution.

When I change to a new PC, then I install Thunderbird as-is. To make it "my" Thunderbird, then I believe the filter-rules, the calendar data and the contacts is the needed information.

Am I wrong here?

When I back-up only "my" data, this makes the back-up smaller.

I'm aware of this. That is the simple solution.

Then why would you want it to make more complicated?

When I change to a new PC, then I install Thunderbird as-is. To make it "my" Thunderbird, then I believe the filter-rules, the calendar data and the contacts is the needed information.

Correct. You simply copy over your backed up profile, and you're done. No messing with restoring individual files, settings, and extensions.

When I back-up only "my" data, this makes the back-up smaller.

The profile is all your data.

My backup sizes may make my question clear: since my day is filled with programming the backup I most need is one of changed text files. These compress very well and the size of the twice-daily run backup is therefore of a limited size. When I included the (entire) profile folder in my backup-rules the result was a 10 to 20 fold increase of the backup size.

My Idea was, that a selective backup of the the information mentioned above, will add just little both to the backup size and the time of the backup.

Actual mail files would be the largest part of the backup, and the rest of the profile is neglectable in relation to the size of the mail part. I therefore don't understand your size problem.

What absolute size is your profile folder, and what is the size of your backup of the entire profile?

What files in the profile cause a 20 times increase of the size of the backup compared to backing up mail only?

Unfortunately I did not emphasize---but stated clearly---in my question to this forum, that the question only concerns the profile folder. So this has now been made.

Here are the file- and folder-sizes concerned:

      (kB)
227 000 E:\proj\Thunderbird\profile
         20  E:\proj\Thunderbird\profile\abook.mab
      812 E:\proj\Thunderbird\profile\calendar-data
          ?  E:\proj\Thunderbird\profile\Mail\Local  Folders\msgFilterRules.dat
         1 E:\proj\Thunderbird\profile\profiles.ini

(my apologies for the table not being typeset)

So, this table shows not only the size in kB of the entire profiles folder, but in addition the sizes of the files I asked for. Thanks to your questions, I have now (almost) found the files and folders I asked for. The missing item is the filter-data. (I cannot find it neither at the MozillaZine indicated place nor by massive use of grep. But Thunderbird knows where the 201 filter-rules are located).

Summing you see that the files I need are in fact 1/200 of the size of the entire profile folder.

Does this clarify to you, that the twice-daily repeated backup of the profile folder gives a very large backup file? Needless to point out, that, e.g., the precise contents of the inbox or the language-definitions (both parts of the profile folder) is of not much interest?

Thank you for your questions. Their upshot was that I defined an additional backup job that runs only weekly. This backup saves the above mentioned files. This backup corresponds to my needed information and to the frequency with which that information is relevant for me to save.

The missing item is the filter-data.

Filter definitions are per account. http://kb.mozillazine.org/Filters_(Thunderbird)#Export.2FImport

E:\proj\Thunderbird\profile\abook.mab

There may be more to this. See http://kb.mozillazine.org/Moving_address_books_between_profiles

E:\proj\Thunderbird\profile\calendar-data

Note, the calendar data is useless without the corresponding prefs.js file.

Does this clarify to you, that the twice-daily repeated backup of the profile folder gives a very large backup file?

I'm not going to argue about the size. The recommendation to backup the entire profile still stands. How often you want to do that is entirely up to you.