Here's a little gotcha in Firefox 3.0 for web site designers. My website was incorrectly formatted in Firefox 3, but worked fine in Firefox 2. I had some JavaScript code that did the following:

  mainTabDiv.style.left = (mainTabBarLeft-mainTabBarDX)+'px;';
  mainTabDiv.style.top = mainTabBarTop+'px';

With that code in Firefox 3, mainTabDiv was not positioned correctly -- it was at 0. So I put in an alert: and got the following output:

left= mtbl=270 mainTabBarDX=2

Can you spot the problem? The first statement in my source has px; and the second one has px. That is, the failing statement had a semicolon at the end of the string. In Firefox 2 this worked OK. In Firefox 3, the assignment does nothing. So check your JavaScript for stray semicolons if you're having site issues in Firefox 3.