I just experienced this issue for the first time yesterday, after upgrading to Firefox 5. The same "magic" url for a single thread (http://www.flyertalk.com/forum/southwest-rapid-rewards/1229903-southwest-early-bird-boarding-sham-new-post.html) redirected to two different page anchor urls. In FF 5 (on Win 7 Home Premium) it redirected to (<http://www.flyertalk.com/forum/southwest-rapid-rewards/1229903-southwest-early-bird-boarding-sham.html#post16620273>) whereas on FF 4 / Win 7 Starter it correctly redirected to my first unread post (<http://www.flyertalk.com/forum/southwest-rapid-rewards/1229903-southwest-early-bird-boarding-sham.html#post16623396>). On the computer where new-post redirected to the wrong URL, the View First Unread link was nevertheless correct.
I have yet to encounter any intermittent problem with the new-post redirect. It has always worked, and continues to work, on FF 4, and always doesn't work on FF 5.