I have an issue with Firefox if the about:config option “privacy.resistFingerprinting” is enabled. In that case the drop-down window has a random size at each opening. Sometimes large, sometimes small.
IMHO it’s a feature from FF so the extensions can’t guess your screen size which is used for fingerprinting.
We use the window’s screen size properties to determine the height of the window. I suspect @Crocmagnon is right here.
I don’t know the technical details of Firefox but sites somehow determine the correct size of the Firefox client area. At least users need that the drop-down Bitwarden window has a proper size. When the window is too small (as on the screenshot above) it is very inconvenient to work.