I would keep the current version, and for accessing the site that needs Firefox 2 install the portable version of Firefox 2. You can install the portable version on your hard drive, and it will not interfere with the current version. This is a better option because Firefox 2 is no longer supported and does not receive any more security updates. You can use the more secure current version of Firefox for general browsing and just use Firefox 2 when required. You can get it from http://portableapps.com/apps/internet/firefox_portable - see the legacy version links at the bottom of that page.