I have the same issue, on multiple platforms - Windows, Solaris and Linux (Firefox and/or Mozilla on all three, various versions up through 1.5.0.1 on Firefox/Linux). When I enter on
www.986forum.com, I'm not shown as recognized and the "New Posts" link is not available. I figured out a long time ago that the first thing I should do is click on the "Forums" link and presto! I'm now recognized when that page loads. I don't know which side of the SW equation is doing the "wrong" thing... I'm sure there would be some religious arguments on both sides, vBulletin and Mozilla.
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