Force URL link to open with a specific browser
You could use the all powerful open command to achive this using a shell script / apple script. Not necessarily the nicest way, but that's the way I do it. 

open http://apple.stackexchange.com -a Firefox.app 


would be the basic command to open this page in Firefox. You can easily wrap this in a nice to use apple script - let me know if you need to know how!