When Firefox is set as the default browser, "Search in Google" from various locations in Mac OS should perform the search in Firefox. Having Mac OS launch a non-default browser to perform a web search at Google is just wrong.
I'm guessing Mac isn't going to make this easy for other browsers. Then again, they didn't make "set as default browser" easy for other vendors and we managed to get that working.
From John Gruber:
"If you Control-click on a word or a range of selected text and choose “Search in Google”, the search is performed in Safari, regardless whether it’s your preferred browser (or even whether it’s already running)."