re 5: I don't mind the speed at which it scrolls, I just want one scroll wheel tick to be less.
re 4: I do have the keyword 'g' set up but it is still easier to not type g[space] everytime. And yes there is ctrl +k but it is still easier in chrome with just one location.
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.
ad 4: you can create a bookmark, edit its "location" and "keyword" accordingly. "%s" is the placeholder for the text you type. E.g.
Location: http://www.google.com/search?hl=en&safe=off&q=%s Keyword: g
Usage: Type "g whatever" in the URL bar to launch google search for "whatever".
You just need to extract the minimal URL and place "%s" accordingly. That way you can create multiple ad-hoc search engines, like "so foo" -> google search "site:stackoverflow.com foo" etc.