Support for the ‘original’ IE Tab ended with Firefox 3.5 in June 2009. I believe it was through Go Firefox! I learned about IE Tab +. The extension not only supports Firefox 3.6 but also has an integrated version of AdBlock Plus that will work within the IE Tab.
IE Tab + works with Firefox 3.6 and 4.0 (32-bit only).
Update: 03/15/2011 IE Tab + (not to be confused with IE Tab Plus) supports Firefox 3.6 and newer