I don't know of any built-in way to do this. It can be done with a pretty simple shell-script, though:
Just save this file as "google" (no extension), somewhere in your PATH and make it executable (chmod +x google). Now you can open a terminal and type:
/usr/bin/firefox $url &
Note: Use a plus (+) sign between search terms instead of a space. I'm sure someone who knows bash scripting above my novice level could automatically convert spaces to a plus sign.
This might be more effort than you're willing to put into getting search functionality in the terminal, but figured I'd at least throw that out there...