Stop trying to patch the old IE Tab, it won't work for opening a page in an IE tab! IE Tab's author said he won't update it anymore, and has invited anyone to work on the open source code, which "ietab.net" has done, creating IE Tab 2. It's in beta, but it looks and works exactly like the old add-on, and works great in the latest Firefox. Like others have said, it's right here:
You just have to check a box saying "Let me install this experimental add-on", and you have the same add-on we all know and love!