Firefox login problems

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I have login-problems on dropzone.com, related to Firefox. No problems when using Internet Explorer.

In my Favorites, I have bookmarks to my favorite forums, eg:
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?forum=18;

The URL works fine, but:
-1- with Firefox
=> 75% of the times I'm not logged in automatically.
=> If that happens, logging in manually just doesn't work
=> 25% of the times I'm logged in automatically

-2- with Internet Explorer
=> 100% of the times I'm logged in automatically, never had any problems

Yes, I have the "remember login" flag on when logging in. Username & password are correct. Cookies are allowed. I have checked and double-checked every Firefox setting I could think of.

Going directly to http://www.dropzone.com, works fine 100% of the time, even with Firefox.

Any ideas??

Not being able to use tabbed browsing on dz.com feel like flying a round parachute.