Getting Firefox 4 and Firefox 3.6.16 portable to work at the same time

I have installed Firefox 4.0 on Windows XP, but for some legacy applications I need sometimes to use Firefox 3.6.16, so I installed:

Firefox 4 in C:\program files\Mozilla Firefox

Firefox 3.6.16 portable in C:\program files\Mozilla Firefox3

I use 2 different shortcuts on the desktop, and when I click on the icon of one of the 2 versions, it works as expected. Problem is, when I have a Firefox 3 running, I click on the Firefox 4 icon, it opens a new instance of Firefox 3 instead. I cannot get the 2 version to be open on my desktop at the same time. Is it possible to get the two versions to run at the same time?

Solution

You need to use the -no-remote command line switch. You only should use this switch with the version that is not your "regular" version, e.g. Firefox 3, or else opening links from another application will either fail or open yet another instance.

Also make sure you use different profiles and not share them. See the -P {profile} (named profile) and/or -profile {path} (path to the profile). E.g. use the default profile for Firefox 4, but another one for Firefox 3.

Putting both together:

Firefox 4 shortcut: No modifications, use the regular one.
Firefox 3 shortcut: something along the lines of:

"C:\program files\Mozilla Firefox3\firefox.exe" -no-remote -profile C:\firefox3profile

Using -no-remote and profile switches will not only enable you to run different versions at the same time, but also multiple instances of the same version using different profiles, and with that different sets of preferences, cookies and extensions.

There is also a standalone Profile Manager that mozilla developed recently, which might help you manage different versions and profiles more easily.