"Compatibility View" add-on for Firefox?

Does anyone know if there's an add-on for Firefox 4 similar to the compatibility view in Internet Explorer? I upgraded to FF4, but now I don't know what my sites look like in FF3.6, which would probably be handy to do (I know I use the compatibility view quite a bit in IE since users of that tend to be slow to upgrade for some reason...)

I haven't seen an add on for that yet. There might be one, but I wouldn't think that it'd be quite up to the same "standard" as the way that MS does it (I do feel dirty for saying that!!).

As a personal "feeling" I am not quite so worried about users still on 3.X versions of FireFox. FF users are a lot like chrome users, in that they are about a million times more likely to upgrade their browser version then IE users that wouldn't upgrade unless forced to.

I know that there's still a lot of FF users out there that haven't upgraded to 4 yet, but when 4.1 comes around there will be a pretty big swap and you'll see that users are leaving the older versions behind.