You probably want Cobrowse.
"Essentially, this extension is a collaborative web browser. "Collaborative" is a bit of a stretch, so we should actually think of it in a leader/follower sense. With cobrowse installed, one user can start a "server" that will accept incoming connections (currently on port 5000) from folks that have chosen to connect via "client mode".
The server browser then sends every web page navigation change to every client. It even works when changing tabs!"
Unfortunately it's for FireFox 2.x only, maybe you can persuade the developer to upgrade it to be compatible with 3.x
http://cobrowse.mozdev.org/Or otherwise you could something like Gotomypc or gotoassist to share your desktop.