At first I tried to login into this forum via newest Firefox on Windows.
This is not possible.
A login popup with rotating circle appears which rotates forever.
Whats wrong?
I had to use Chrome for this post.
I am writing this using Firefox 91.5.1esr, and I had no problem with logging in.
Did you try starting your Firefox with all extensions disabled?
Just FYI, at least in order to be able to download attachments, I have to set network.http.spdy.enabled.http2 to false when using Firefox, which means that in browsers where I cannot disable HTTP2 like in Safari, I cannot download attachments here at all ;D
But login, indeed, works fine for me, even with HTTP2.