I created a bitwarden account and verified it successfully. I can access my vault with a web interface, but when trying to login through a firefox add-on or a mobile app the error “username or password incorrect. try again” appears. I double-checked the credentials a thousand times and made sure my sanity is still intact by copy-pasting with my text editor. Furthermore, firefox as well as my (non third-party) add-on are up-to-date.
Turns out there is a problem with the selection of the region. During creating the account I selected “EU”, but when logging in, I have to select “US”. I recreated this behavior by creating two additional accounts in the EU and also having to login in the US region afterwards.