When you install a release candidate it will be default replace the previous version. To stop this from happening use the Custom installation option and specify a different location to install Firefox 4.

If you want to use both versions, you need to create a new profile for Firefox 4 (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)

You could use the Firefox Portable edition and install it to your hard drive, which would automatically be completely separate from any other version of Firefox you have installed, including the Profile.

The 4.0RC version should be available the same day as the RC version is released by Mozilla, but usually later in the day or the following day.

http://portableapps.com/apps/internet/firefox_portable/test

Thank you TonyE & the-edmeister for the feedback.

You're welcome.
Please click the Solved button next to the answer that solved your Firefox support issue.