When Firefox is open, you can use

Help > About Firefox

to see what version you're running. Your Firefox reported to this site that it is Firefox 13, which is obsolete and vulnerable, so you should check this at your earliest convenience. That dialog also has an update button.

If you want to maintain older versions of Firefox for testing, you could use a "Portable" version built by an independent third party. Although a Portable version still cannot easily run simultaneously with your main install, it will have its own settings folder, so you can carefully control your test environment(s).

http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./

That said, most people run the latest version of Firefox unless support was terminated for their OS. There probably are statistics on that somewhere.

You can install multiple Firefox versions via a custom install in different installation folders.

• https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/

If you want to test future development versions:

• http://www.mozilla.org/en-US/firefox/channel/

See also (Older versions):

• https://developer.mozilla.org/en-US/docs/Firefox_20_for_developers

Do a custom install and install each version in its own program folder to use multiple Firefox versions.

• https://support.mozilla.com/kb/Custom+installation+of+Firefox+on+Windows
• http://releases.mozilla.org/pub/mozilla.org/firefox/releases/

Make sure not to start Firefox after the installation has finished.
Create a new profile exclusively for each Firefox version.
Create a desktop shortcut with -P "profile" appended to the target to launch each Firefox version with its own profile.

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox
• http://kb.mozillazine.org/Bypassing_the_Profile_Manager

Use the -no-remote command line switch to open another Firefox instance with its own profile and to run different Firefox instances simultaneously, but do not use this with the default browser and the default profile.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile