I personally have not had any issues with setting Firefox, Opera, or Google Chrome as my default browser on any Window 7 system I have used. But it does seem that some people may be experiencing a problem — after setting Firefox as the default browser and after a reboot, Windows 7 reverts back to Internet Explorer as its default. Now before I proceed with the fix, let us put the conspiracy theories to bed. This most likely is an issue with a bad plug in, patch, or some other glitch.
After doing a little searching I found a last resort fix that may solve the problem. (Notice the word may.) No guarantees.
Go into Control Panel, and select the icon Default Programs. This will open the following window:
Click on Set program and computer defaults. A second window will open that looks like this:
Click on Custom – the two double down arrows on the right side that will bring up this window:
Take a look at the options titled Choose a Default Browser. The check box will normally be next to the Use My Current Web Browser option. You will want to change this to the browser you’d like to use, i.e. Firefox. Make sure Enable Access To This Program is checked. Reboot and check and see that Firefox is, in fact, your default browser.
I hope this helps those who are experiencing this problem.