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To set FireFox as the default browser for Windows, open firefox and go to:

Tools -> Options -> Advanced -> General (tab)   and check the box that says "Always check to see if FireFox is the default browser on startup", and then click "check now"

If firefox says something like "Firefox is already the default browser" but another browser loads sometimes, use this method to force FireFox as the default browser:

1) Open "Run..." in windows (start menu, type run, press enter)

2) Copy / paste this into the run text box:    firefox.exe -silent -setDefaultBrowser
(and then press enter or click OK)

reference page:  http://kb.mozillazine.org/Default_browser

PS:  Now that Google Chrome is faster than FireFox, here's how to set Chrome as your default browser.

How To Make Google Chrome Your Default Browser

If you like how Google Chrome works, make it your default browser! By doing so, future links you click will automatically open in Google Chrome. Follow these steps to set Google Chrome as your default browser:

1. Click the wrench icon wrench icon on the browser toolbar.
2. Select Options (Preferences on a Mac).
3. Click the Basics tab.
4. In the "Default browser" section, click Make Google Chrome my default browser.

from Google Support:  http://support.google.com/chrome/bin/answer.py?hl=en&answer=95417