Firefox 5 has been released only a few days ago and I have already written about a problem while upgrading Firefox 5. There are a lot of addons that are incompatible with Firefox 5 while they work fine with Firefox 4. In this situation, if you want to run both Firefox 4 and Firefox 5 in your computer, it will be impossible to install both of them.
The simple solution for this kind of problem is having a portable edition of the browser, Firefox 5 portable. Install it once in a single folder and then take the folder anywhere you want. Firefox will run with all your settings preserved. Working with the portable browser is the same as the installed version. You can install the plugins and addons and they will be installed in the same installation folder enabling you to take the folder with all the files and settings anywhere you want.
Firefox 5 portable comes with the same features as in installed Firefox 5 but it is not an official release. It has been released by Portableapps.com
Download Firefox 5 Portable