Likely. There are several threads reported here where Firefox can’t authenticate or remember the Plex credentials. Users have tried different settings and what works for some doesn’t for other. I’m not able to reproduce the problem myself.
Are you still on 1.22? If you upgraded to 1.23, there was a bug that effected some servers from not being able to access plex.tv. This should be fixed in the latest public or beta. Try updating even if you are 1 version behind.
