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:
open -a FireFox "http://www.google.com/search?q=$1"
as the shell script text, then save it as "Open with Firefox" or whatever you like.
follow this summary from this thread:
Download and Install OnMyCommand/OMCEdit
Click on "Download Commands" then "Download"
Go in "Command Library"
Choose #22 and click on "Append to Command"
Change the name if you want (in "Command Name")
Write " Top Level " in location if you don't want a sub-menu, or choose a name for the sub-menu ("In On My Command" is the default)
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.