'Safe Mode' would certainly not have resolved the problem but it does remove unnecessary drivers and programs for diagnostic purposes.

Am I right in thinking that, when you started TB in 'Safe Mode with Networking', there was no appreciable change in the problem?

thank you for your quick answer. You are right, also in safe mode it is the same annoying problem. At the moment I think of renaming the profile folder, completely deinstalling TB and try a new TB-installation. I have a bit fear of losing data... Greetings from Germany Michael

I have not had any experience with this sort of problem but, having had 'hangs' in other software, it is often a software/OS conflict or a problem with the program - that usually resolves itself.

You've eliminated the likelihood it's Third Party software by starting in safe Mode.

You could do a fresh install after treating it like the moving of a profile to a brand new set up, backing all the files up to an external HD - https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles

However, before a reinstallation, it would be a good idea to try and over-installation - but make sure you do back up all your profiles and data (as above) and use only the installation file that is the same version as the one you are currently running.

And, before an over-installation, back up the data/profiles and then make sure that you are running the most current version - Help/About Thunderbird will show whether there is anything to update.