Is it possible to add rpm.pbone.net or rpmfind to apt or yum repositories?
No, it isn't. These are search engines like Google or others. Try to learn about installing software in Linux, go to Google Linux
, search for "software installation howto". You will get a loads of useful links, just read the docs because you'll be in trouble without this knowledge.
Now to your specific question: read this guide
to understand how to compile a GTK, install it and then get a Firefox installer from Firefox site
and install it. There is installation instructions inside every package too... Good luck.