Is there a plugin that lets you switch firefox versions

Is there a plugin, that lets you switch firefox browser mode, the way you can do in IE?

I want to see how my CSS renders in older ff versions. (To be exact, I want to see, how it renders on ff for Mac, but I do not think this is possible to emulate. So I want to see, if the error appears on older ff versions, too)

Answer

What IE does when switching the browser mode is changing the user agent string (and possibly using some older rendering approaches).

Switching the user agent string in Firefox can be done with the User Agent Switcher extension.

After installing it, import a list of user agents from this forum post. You'll then be able to switch the user agents from the menu.