That is a feature of the beta, being able to coexist with the release level. But that may not work for you as the beta is now beyond 115. My suggestion is to click to website of each addon to see if they show support for 115.

Most extension just list "v102 and up", and have no specific "v115 support" opt-in. I suppose many older extensions won't directly get updated, so it would be vital for me to test the installation. If my mails don't work, I can't work and I can't earn money :-D

If I cannot run them in parallel, I guess I should be able to create a second useraccount on my macos (I don't have a second computer) and install the new version just there, and choose a custom new profile (like I would for a parallel v115 install)

Install TB 115 to a separate folder, then at the end of the installation, don't launch it. Start Profile Manager with TB 115. Uncheck 'Use the selected profile...', create a new profile with a distinct name such as TB 115, then add your accounts to the new profile. This way, the 102 and 115 setups are separate and independent, as if they are on separate computers. When you launch 102 or 115, the Profile Manager will appear for you to choose the appropriate profile.

The current tbkeys-lite supports 115.

Thank you! Indeed this works with two profiles alongside, thank you.

Darko seems to work flawless, as does tbkeys-lite.

Sadly "Mailbox alert" for custom mail sound notifications per folder/account is not compatible, and neither is custom sorting of accounts in the left pane. I hope Thunderbird somewhen natively supports this or a new extenion comes along, those two are vital to me, so I'll need to stick to v102 for the time being.

For custom sound notification, try FiltaQuilla 4 and create filters.

Sorting of accounts can be done by drag and drop in the left pane of Account Settings (also in 102).