Could this be done by running e.g. a portable instance of Firefox alongside a regular install? I read that a while back some people were doing this to use/test Firefox 2 and 3 concurrently, but I never gave it a try.
Currently, I keep a clean Opera installation solely for high-importance secure transactions, but I like some of the features that (I know, I have to trust them) Firefox extensions add to Gmail. I also don't want to be running Gmail through that clean Opera installation; I want to restrict my use of the latter to access to known sites and trusted content.