A couple of methods:

You can install them both but there are a few steps to take.

When you install the second version of Firefox use the custom installation option and choose a different location to install Firefox.

In order to use both versions, you need to create a new profile for the second version of Firefox (Firefox stores user data in the profile). Using the same profile with different versions can cause problems. Once you create a new profile you can copy certain data such as bookmarks and passwords between them. You can also use Firefox Sync to keep the profile synchronized. Finally, you will need to set up shortcuts to launch the correct profile for each version.

The following links show how to do this:

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
• https://addons.mozilla.org/firefox/addon/firefox-sync (only needed for Firefox 3.6, it is built into Firefox 4)

An alternative method is to install the portable version of Firefox on your hard drive, it will automatically create its own profile folder and will not interfere with the other version of Firefox. You can get it from http://portableapps.com/apps/internet/firefox_portable - If you want to install a portable version of 3.6.17, the link is near the bottom of that page.

THANK YOU AM I CORRECT THAT THIS WORKS BOTH WAYS 3.6 TO 4.0 AND 4.0 TO 3.6?

You can use the method to install any 2 versions of Firefox. It does not matter which one is installed first.

Yes, you can use profiles, but non-techies will be completely overwhelmed trying to do that, and it's easy to destroy your whole configuration if you're not careful.

What I do is install a known stable version (3.6.15 works for me) while logged in to the admin account. I make sure a shortcut is created on the desktop, and create it manually if need be. (Right-click on the desktop, select New->Shortcut, and type in the installation pathname.) This shortcut should get propagated to the public folder so that all users will see it on their desktops. (And can't get rid of it, sadly, but hey, Windows ain't Unix.) I manually change the name of the shortcut from "Mozilla Firefox" to "Firefox 3.6.15". Again, this change will be reflected on all the user desktops.

This serves two ends: (1) you don't want an administrator running unstable apps, and (2) it gives all users a stable default Firefox.

The next step is to switch to my personal login (never, ever do non-administrative work using the admin login!) and, as myself, install the new version of FF. Its shortcut then appears on my desktop, and only on my desktop. I manually change the name of the shortcut to "Firefox 4.0.1".

You now have two versions and two icons. You can play with the new version to your heart's content, and, if it misbehaves, you can fall back to the stable version in a trice.

Hats off to Mozilla, BTW, for letting users install Firefox in their personal filesystem space! Many industry giants (are you listening, Adobe?) won't allow this.

It appears this thread started out as a discussion of sync, so let me add that I'm not thrilled with FF4 sync. It eats a lot of bandwidth, enough to slow down the application noticeably (run task manager and watch the core memory expand without apparent limit). Also, I sync only bookmarks and preferences, yet I still get messages about exceeding storage space on the server. (Presumably, this means I'm not being synced, yet I receive no warnings about that.) Caveat user!

Another approach would be to have:

• a standard installation of one Firefox variant to Drive C:, and a "portable" version written to a second hard drive, or a "memory stick"; or

• a "portable" Firefox 3.6 installed on one "memory stick", and a "portable" Firefox 4 installed on another "memory stick", or a different folder on the same "memory stick".

Alternatively, for "memory stick" read "flash memory card", and use a USB card reader to access the memory card as if the memory card was another disk drive or memory stick.

I believe I have accidentally attempted to run two instances of Firefox on a Windows machine [XP], by repeating a left-click on a sluggish machine, but found that Firefox, unlike Notepad and other applications, will not allow two Firefox processes to run simultaneously. So it would seem that you would not be able to do multi-tasking, with FF 3 running one window, and FF 4 running another.

"Portable" Firefox? See:

Portable software for USB, portable and cloud drives

• http://portableapps.com/
• http://portableapps.com/about/what_is_a_portable_app

Free Portable Software to Carry with you on a USB Flash Drive

• http://www.pendriveapps.com/

Move away from the Windows/My Documents trap; keep Drive C: for the operating system, and put your data and personal software on another disk drive altogether, ready for when Windows/other OS dies, you can't get access to Drive C:, and re-installing Windows/other OS, or over-writing with an image of a working installation, are your only options!

THANK YOU FOR YOUR HELP. I FOUND THAT I CAN INSTALL BOTH 3.6 AND 4.0 AND USE THE SAME PROFILE WHICH ALLOWS ME TO COMPARE THEM ON THE SAME APPS. THIS IS WHAT I REALLY WANTED TO DO. AS FOR YOUR ADVICE ABOUT A SECOND DRIVE, I STARTED DOING THAT WHEN I HAD A BUSINESS AND WANTED TO HAVE MY ACCOUNTING SOFTWARE AVAILABLE WHEN I UPGRADED COMPUTERS. WITH TODAY'S LARGE DRIVES (750 GIG AND UP) IT SEEMS SINFUL TO KEEP IT JUST FOR THE OS. WHAT I HAVE TAKEN UP INSTEAD IS TO HAVE A SECOND HARD DRIVE THAT I POP IN MY NEXSTAR HD DOCK AND USING ACRONIS SOFTWARE MAKE A COMPLETE CLONE OF MY HD ABOUT ONCE A MONTH. WITH THIS I CAN BE SURE THAT IF I HAVE A DRIVE FAILURE (AGAIN) I WILL NOT HAVE TO SPEND THE TIME AND MONEY TO LOCATE THE OS AND PROGRAMS AND DATA I HAD. USING ESATA I CAN BACKUP 750 GIGS IN ABOUT 45 MINUTES