It is possible to do this. However it is not normally something done other than in extreme circumstances such as running compatibility tests in virtual machine.

Later versions tend to have improvements: more features use less memory and are faster, but most of all are safer. Using an outdated version puts your machine and your personal data at greater risk due to all the potential exploits that have been fixed.

If you just have one application that is impossible to run in anything other than Firefox version 11, it may be worth installing two versions and using the latest release most of the time and the legacy unsupported, insecure version 11 as little as possible, and preferably offline from the internet.

May I ask what the reason is for requiring version 11, because possibly there is a safer workaround. If it is regarding software compatibility what is it that you are running/testing ?

Like John said, there are so many security issues in Firefox 11 (read https://www.mozilla.org/security/known-vulnerabilities/firefox.html, Firefox 11 is vulnerable to almost everything fixed in Firefox 12 and later) that it's simply unwise to keep using it, not to mention you aren't getting new HTML technologies, performance improvements, bug fixes, etc.

You can install a portable Firefox (ESR) version to access websites that do not work with the current Firefox release.

• http://portableapps.com/apps/internet/firefox_portable
• http://sourceforge.net/projects/portableapps/files/Mozilla Firefox, Portable Ed./
• http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20P.E.%20Legacy/