Hi,
I'm a Firefox user myself, but I do not experience your problem. I'm now logged in since several weeks. I'll investigate if there is any problem with the cookie URL settings or similar tomorrow morning when I'm back in the office.
Do you use any privacy-enhancing features of Firefox? Maybe accidentally set our domain to keep cookies only for the current session, or totally blocking cookies from our domain? (I know you said you checked you settings, but just to be sure

)
Sorry for the inconvenience.
Best regards,
Martin