i m using linux mint firefox version 94.0 (64 bit)

Works for me. see screenshot

firefox --private-window https://mozilla.org

yes why not. but on my desk it doesnt work :-( it just opens a normal browser window

Try it with http or https in front.

this i tried already before raising this question here.

i tried it with firefox on my raspberry (with latest os), and there it works as expected.

So this is not a browser issue right? I would try your OS forum.

no. i guess its a browser issue. its mostlikely an issue in the area of the combination browser / distribution (plattform).

since its clear that its not related to usage (wrong input by me), and because i have found other reports related to similar problems (also on windows) i dont follow up here in a different forum.

i keep this question (issue) open until it works on my plattform. thats all.