I second that, although I do it the other way round... Using WebKit, and the Mozilla-based Firefox whenever something seems strange (rare cases).
(And strangly, in these rare cases, Camino tends to work better. Since Camino uses Webkit as well, I guess in the end it does not come down to a Webkit / Mozilla issue.)
Due to (thou rare) incompatibilities between browsers and web-sites it is good to have a few different browsers. So if something doesn't show up "right" you can use another browser to see if "fault" is on your browser or web-site side. Which is what I do.
Regarding preferences - I used to be FF user. But I like Safari's search on the current page so much that can't use FF anymore 🙂