Cmd+Shift+L searches for the selected text in Google using Safari. Can I change this to use Firefox instead?
You can create a new service with automator, use the "Run Shell Script" action, and use:
as the shell script text, then save it as "Open with Firefox" or whatever you like.
follow this summary from this thread:
What you need to do is create a new Automator service. Unfortunately, Firefox's AppleScript support sucks, so you'll have to do it manually.