If your problem is specifically with "define", you could set up a bookmark to http://www.google.com/search?hl=en&safe=off&q=define:%s with the keyword "define:". Just checked and firefox doesn't complain about the colon in that case.

For the more general case, I agree that firefox can be a bit annoying about treating any address with a : as some kind of protocol. It occasionally bytes me when doing things like trying to google a Python error via copy-paste.