You're 10 security updates behind - 3.6.16 was released yesterday.

Mac = only one version of Firefox at a time.

Windows = Install Firefox 4.0 using the Custom Installation option, in a uniquely named folder - like /Program Files/Firefox 4.0/ - don't let 4.0 run after the installation procedure is complete.

Create a new profile exclusively for the 4.0 beta version and create a desktop shortcut with -P "profile" appended to the target to launch that profile.

• http://kb.mozillazine.org/Testing_pre-release_versions

• 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

Thanks.

I have 3.6.16 installed (just lazy when I typed in the version #).

> Mac = only one version of Firefox at a time.

Not true. The process is not all that different from how it's done in Windows (this assumes you know where (& what) the Terminal is).

1. Install (i.e., drag) Firefox4.x into a different folder than Firefox 3.6.x (a sub-folder of the Applications folder or wherever else you like).

2. Launch Firefox 4 with the Profile Manager by entering the following in a Terminal window:

    /Path/to/Firefox.app/Contents/MacOS/firefox-bin -ProfileManager

Replace the "/Path/to" part with the actual path (duh) and type a "Return" at the end of the line.

3. Create a new profile for the 4.x version and uncheck the "Don't ask at startup" box, then click the "Start Firefox" button.

Now you can keep icons for both versions in your Dock (or put aliases to them in some convenient location) and when you launch either of them you'll be presented with the Profile Manager and can choose the appropriate profile (if you like you can paste a custom icon onto one (or both) versions to make it easier to remember which is which).

See also:

• http://www.businessinsider.com/how-to-run-firefox-4-and-3-simultaneously-in-mac-os-x-with-multiple-firefox-profiles-2011-3