I'm not sure this is easily possible. What could work is a combination of UI scripting and a dedicated Behaviour for just this purpose. Then, however, I'm not sure whether this is possible at all, because UI scripting requires an application to provide accessibility information - something FireFox doesn't do (although it's interface looks Mac-like, FireFox draws it all by itself).

If there are just a few sites you want to visit, it's probably a much better idea to create a custom action for each, using an AppleScript like this:

do shell script "open -a Firefox \"http://www.iospirit.com/\""

Be sure to check the "globally available" checkbox. You can then add these to the menu for FireFox at Prefs > Menu > FireFox by dragging it from Actions > FireFox > .. on the left.

Best regards, 
Felix Schwarz