Are you really determined to get rid of your existing contacts. Uninstall and reinstall generally fixes exactly nothing. However folk frequently manage to loose all their historical email and contacts by doing so, and then look for someone to fix their fix.

If you are determined delete the profile folder. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

My problem was a bit different from just deleting my profile. I solved the problem with Revo uninstaller and deleted all files and folders which were connected with Thunderbird in any way. As I found out there were a lot of registry-entries and dlls and files left from Version 115 ( two Updates before) . The update process had not worked properly then. Afterwards I installed Thunderbird new and it worked. As I work with IMAP I did not loose much if any ( still synchronizing). What do you suggest for updating Thunderbird to avoid such a problem? Deinstall old Thunderbird first and do a new install?

Personally I would suggest deleting Revo and forgetting you ever heard of it, so I am probably not the one to ask for advice on fixing issues. My experiences here are folk who use Revo generally have ongoing issues.

If your issues were post an update, it was probably a rather long way around fixing your antivirus or firewall. That is what I am thinking based on the seriously minor amount of information you supplied. You never actually said what the problem was!

I can not begin to offer advice to prevent a future issue as I really do not know what you issue was. I like you have no idea what went wrong and your fix did not uncover that, so I am left guessing based on the thousands of post that I have seen over the last 10-15 years of folk not getting mail post update. I have seen perhaps a few instances where the issue was Thunderbird but they were rare and you have said nothing to indicate the issue was actually Thunderbird, so I assume your issue lies elsewhere.

When I read your post, my first thought was that you may be looking at revo uninstall, which shows remnants of prior version as though it was still installed - when it is not. In general, these are harmless, as I regularly see revo uninstall showing i have two copies. My guess is your problem and the revo uninstall display are a coincidence. Reinstalling the bad release would likely have resolved the issue. Telling revo to uninstall the old version also causes it to remove components of the current version.