Excellent Rated 5 out of 5 stars
This is an extension every Firefox user must have. The only real problem with this is that it has a weird effect on jump forward, but it isn't that big of a deal.
Do you know what would make this extension even better though? If there was some way of applying Firefox's tools (such as spellcheck) on an IE tab in Firefox. It would be difficult to "Firefoxize" IE, but it would be cool.