Firefox 3 is one of my most favorite browsers ever, Firefox 3 sports a better bookmark manager and is supposed to render web pages more quickly than Firefox 2, and all in all a superb visual design, but to use it I had to undergo lots of hardship, most of the themes and very useful plugins will not work, and some plugins throw errors.
So I was left with no choice but to switch back to Firefox 2
But Little googling helped me to come up with this solution.
Here is how to use Firefox 3 alongside Firefox 2
1. Download the Firefox 3 Portable edition from – Sourceforge.net.
2. Just in case you do something wrong, backup your profile
type %appdata% in your explorer addressbar and open Mozilla folder –> Firefox –> Profiles and inside you will find a folder named xxxxxxxx.default, copy that folder to some location where its safe.
3. Extract this Firefox 3 installer to some new folder like C:\Firefox3 or wherever folder you like.
4. Close your Firefox 2 browser and run the FirefoxPortable.exe file available in the above folder mentioned in step 3.
5. That’s all. enjoy 
