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 separate profiles for each 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 is to use the portable version of Firefox for the second version of Firefox. You can install the portable version on your hard drive, and it will automatically create its own profile folder and be independent of the other version of Firefox. You can get the portable version of Firefox from http://portableapps.com/apps/internet/firefox_portable - for older versions of Firefox see the legacy versions section near the bottom of that page.

where download 3.6 now have 4...?

Firefox Setup 3.6.10.exe I have this in my hdd... install this or download newer?

you say: "Using the same profile with different versions can cause problems. ..... Finally, you will need to set up shortcuts to launch the correct profile for each version. "

installing 3.6 in separate folder, guarantees that 4 work without problems so, configure only 3.6?

portableapps.com and u3,

usb sticks can used interchangeably are same technology?

You can get Firefox 3.6 from http://www.mozilla.com/en-US/firefox/all-older.html