Hooray! Rated 5 out of 5 stars
Yay! Extended for FF3!
Be aware of the toolbar icons (View->Toolbars->Customize) as they are a must.
Small note: with the XUL editor, if you open it in a new window, it seems to have fewer privs (had problems testing a controller).
An XHTML 1.1 editor might be nice too to get immediate well-formedness errors, though one can kind of use the XUL editor for that (though not with real HTML styling).
Small bug (with FF3?): Might not be ext's fault, but reloading chrome (with a lot of windows open at least) caused FF to crash, and I had problems in FF2 with reload chrome causing multiple windows to be opened when reopening Firefox later.
Again, awesome work, thank you!