Hi,
I updated to Firefox 3 today and I saw the same problem you mentioned, the ExtJs Tab Panels were displaying too wide.
I still believe this is a bug in Firefox 3 because 100% width should not overflow its container. However I fouund a workaround.
If you add this in your style-ext-override.css file in your skin folder:
.exttabs { max-width: 800px; }
it will limit the width sufficiently for most skins, for a couple of them I had to use 750px or 740px
Best,
Joe