I keep having problems too. Usually I have to go up to my tools and clear history and then log back in ... then if I click the gallery I can usually get in. So I just keep doing that, it seems to work.
Might have to try the remember me function .... maybe that will work too.
Firefox seems "quirky" some days ...
Edited to add: Tried the remember me function, didn't work .... but if I click on one of the threads and then go to the gallery link I seem to be logged in, so that did work.
Thanks Carol for the tip!