Yes you could do what you want, ALTHOUGH you couldnt call it Firefox and of course you would have to give full credit to where you forked the code from (IE the Mozilla Foundation).

So what do you want to change in Firefox as most things are changable by plug-ins.

If you really do want to change functionality it may be better to go back even further and design and build a completely new browser just based on the Gecko HTML parsing engine (The one that powers Firefox).

Jen