How to use a password manager in mullvad browser?

password manager like keepass is impossible to use in mullvad browser right?

There’s nothing stopping you from installing the official KeePassXC Browser Extension, but doing so may make you stand out among other Mullvad Browser users as most will not have the extension installed, which defeats one of the main privacy protections of making all Mullvad Browser users look the same. Personally, I use Brave for logging into (and staying logged into) accounts (I have my password manager extension installed there), as there’s no point trying to blend in with other users if I’m logging in anyways.