You use softwares with auto "cleaning" settings like System/Registry Cleaners or Antivirus/Security Software?. it is a public computer?

It is my personal Computer. I used a registry cleaner once but a long time ago and it isn't installed on this system anymore, so no active cleaners running. As for antivirus, only the standard windows defender is active.

Edit: Before I reinstalled there was stuff inside the download history, but it was months old and newer downloads wouldn't show up after restarting firefox. I suspect its places.sqlite corrupting but why and even after clean reinstall?

You can check the current places.sqlite file

Type about:support in the address bar.

Scroll down and find the Places Database section.

Click the button Verify Integrity

After it runs, check the results. If it reports errors the file is corrupt. If not the issue might be an external factor preventing the save on exit.

the places database check doesn't throw any errors.

I'd have no idea what is causing this problem other browsers keep the history perfectly fine, firefox just doesn't want to.

Okay it is an error with that specific website and firefox, downloading from the editor wont keep the downloads in the history after a restart.

Downloading from the main menu does keep it in the history but downloading another one from the main menu will kick out the old one and keep the new one. Tested it with a friend and it shows the exact same behaviour

Does the Downloads list (Ctrl+J) shows all of the saves during your session, the problem is only after restarting Firefox? I don't think Firefox should be "de-duplicating" same-named downloads either during your session or at shutdown, but maybe there is something I'm not aware of.

so im downloading the part "spacer.stl" it gets renamed to "spacer(6).stl" to prevent duplicates in the download folder if I download it another time it'll rename to "spacer(7).stl", they're both shown in the download list until I restart. Only "spacer(7).stl" is left in the list, if I now download it another time, "spacer(8).stl", number 7 and 8 will be in the downloads list until next restart where only "spacer(8).stl" is left in the list. The naming doesnt play a role, with the option to choose file location and name if I rename the last downloaded file (example.stl), after a restart only "example.stl" will be left in the downloads list. I only tested this in the same download location not two different ones.

This only applies if I download from the main menu on tinkercad, any downloads from the editor menu will be gone from the downloads list after a restart.