Firstly note:
Do not uninstall the existing version, and certainly do NOT use the delete settings option, that will delete all the important profile information, bookmarks and passwords, in ALL installed versions.

Yes, I run both on the same computer. You will need to create a new separate profile. You should make sure the new profile is created in its own empty folder. Next do a custom install of Firefox 4 putting it in a separate folder so that it does not overwrite the original installation.

Once installed make a shortcut that uses -P argument to point to the correct new profile. You are then able to choose which version to run.

The new version will not then have the same bookmarks as the existing version, but you can use sync to solve that problem.

See

• http://kb.mozillazine.org/Testing_pre-release_versions#Installing_multiple_versions
• http://kb.mozillazine.org/Command_line_arguments

Thanx. Shall try and let you know.

One more point, John, your reply was to install 4.0 over the 3.0XXX. I have the 4.0 installed and am trying to install the 3.6.16 alongside. It shouldn't make a difference, I assume. Correct?

Yes, that is correct. You only need to make sure to do a custom install and that you do not launch Firefox 3.6.x after the installation has finished.

It is better to create a shortcut for each version that specifies the profile (-P "profile") and set the profile for the default browser via the profile manager to prevent problems if you click a link in an external (mail) program.

See also:

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