Or you can just have both versions installed, as I would imagine they would do so in different directories. It wouldn't make much sense for SporkWeasel to be installed in the "firefox" directory. I've got both Firefox 1.5.0.7 and firefox 2.0 rc2 installed. Which one gets called is determined by a symlink.
As I recall, you can run Firefox from anywhere you want. I've run it from my home directory before. All you have to do is copy and paste shortcuts for your plugins. Works just fine.
Heck, you could probably even put it on a jump drive this way...in fact, I think I did that once, too.