IRsoft - Software and more by Ingmar Runge

1. Download the installer for the release you want to try out from mozilla.org and start the setup procedure.
2. Choose "Custom" when you are asked for the "Setup Type".
3. Create a new folder named something like MozillaFirefoxBeta in your Program Files directory (or anywhere else)...
4. ...and choose that folder when the setup askes for the "Install Folder":
   
5. Procede with the installation as usual. Be careful if you got a shortcut on your Desktop with the name "Mozilla Firefox" to start your "normal" Firefox. Rename it before you finish the installation (you can e.g. add "(Main Browser)" or something to the shortcut's name. You also want to uncheck the options for creating shortcut's in your start menu folder and in the quick launch bar.
   You don't have to do that if the installer you are using is installing a "code named" release.

You need to change some things to make Firefox accept two different versions running alongside without interfering each other.

1. Right click the new Desktop shortcut the installer created and choose Properties from the context menu.
2. Add -ProfileManager to the command line:
   
3. Click OK.
4. Right click your "My Computer" icon and choose Properties or open the System module from your Control Panel.
5. The "System Properties" dialog opens. Switch to the Advanced tab and click the "Environment Variables" button near the bottom.
6. Click the upper "New..." button (the one in the "User Variables for <UserName>" frame) and fill in the fields as follows:
   
   The lazy ones can copy and paste the name from here: MOZ_NO_REMOTE
7. Click OK in all three open dialogs.

Firefox offers the possibility to create and to manage so called Profiles. A Profile contains all your Extensions, Bookmarks, Skins and the rest of your configuration. It is very important to have a profile separated from your main profile for testing releases.

1. Double click your new shortcut. A window looking similar to the one in the following screenshot will appear:
   
2. Click the Create button on the left, enter a name for your new Firefox Profile (like, "Test Profile" or something) and confirm the creation.
3. Then select the new profile from the list and click the Start button.

Congratulations, you are almost done! You can now start to explore the new version. In the next section I'll show how to create a copy of your old profile to use it in the new browser without risking damage to the old main profile.

We created a new profile for testing in the following section. We will now copy our "old" profile (from our main Firefox installation) to use it in the testing release.

1. Shut down ALL of your Firefox instances!
2. Hit Windows+R or choose Start -> Run and enter (copy/paste) the following line:
   %appdata%\Mozilla\Firefox\Profiles. Click OK.
3. An Explorer window will open containing at least two folders. Both folder names consist of some cryptic random string followed by a full stop and the profile's name. There will be a folder ending in .default and one ending in .TheNameYouEnteredForYourTestingProfile.
4. Open the .default folder, hit Ctrl+A, then Ctrl+C. Then switch to your testing profile's folder and hit Ctrl+V. Click yes when prompted to confirm files being overwritten.

That's it. The next you start your testing installation with the testing profile, Firefox will have all your Bookmarks etc. and check for Compatibility updates for your extensions.

Probably, most of your Extensions will have been disabled by completing the last section.

1. Download and install the "Nightly Tester Tools" Extension from addons.mozilla.org. (Use right click and Save As to save the .xpi file to your desktop, then drag the .xpi file from your desktop into the Firefox window and drop it there. Confirm the installation and restart Firefox)
2. You can now carefully enable your "old" Extensions in the Extensions dialog through the "Make compatible" context menu or the "Make all compatible" button.
3. Restart Firefox.

Every now and then, there is a "portable" release of a testing version available on portableapps.com. Portable means it doesn't need to be installed and it's saving all its configuration inside the directory you unzipped it to.

To run both really simultaneously, you may want to go through the "Environment Setup" section of this article too, starting at step 4.