Hello there,
I just registered myself and I couldn't login into Minecraftforge community with Firefox.
The login message says: "You were unable to login. Please check your cookie settings."
I checked settings in Firefox, everything is fine. On any other website, everything works. Just not here on Minecraftforge.
You might want to have a look at this problem. For me it's quite annoying since I only use Firefox.
Thanks for your help!
Some technical details firefox shows in the help tab: