I have this since upgrading to www-client/mozilla-firefox-3.0.5. When I go to https://forums.gentoo.org/ I get the dreaded security certificate dialog, and after several clicks I try to click "Confirm security exception" and it just hangs there doing nothing. I'm using the same profile as I did for the previous version, where I had already added that exception. I tried upgrading to the 3.1_beta2 in mozilla overlay. There it immediately takes me to that website, without asking for a new security exception. Problem with that version is that it crashes within a minute. If you have some suggestions for debugging this, let me know and I'll do a debug build.
Works with a fresh profile. Thanks to darkside for the tip.