There is not that much difference between Firefox 4 and 5.
You can consider Firefox 5 as a minor update despite the major version change.

• http://www.computertechtips.net/64/make-firefox-4-look-like-ff-3-6/

I loved Firefox4. FF5 gives me troub le every single time I use it. I cannot have two copies of Firefox running at once, nor will it come up if anything else is using the Internet. It refuses to go to my home page, which is Google. It is forever giving me a blank page instead of a home page. NOTHING about it works very well. How can I get back to Firefox 4? Or must I change to a different browser?