Firefox 2.0 has this built in now. In the preferences, Main section, you can choose what Firefox shows when it starts up: blank page, home page, or tabs and windows from last session.

Tested and stable for everyone!

Now, admittedly, I still use TMP, but for other things (lock/protect tabs, multiple tab rows, controlling how links open, and all the other lovely tweaks it lets m have), as the main function I used it for has been Done.