I had this problem which was leading to much frustration. I tried some of the suggested fixes and tried downloading the Beta 4 version, which caused many more problems than it solved. I ended up uninstalling all versions of Firefox via the Control Panel, Add/Remove Programs. Just watch closely during the process, and uncheck the box which offers to uninstall off of your bookmarks. It is checked, so you have to actively un-check it when it appears, so you will not lose all your bookmarks and add-ons. The Beta had way too many issues for me to handle, but I went back and re-installed only the 3.6.15 version, and all my bookmarks and add ons are saved. I found that in Firefox 4 (Beta) almost none of my add-ons were compatible, so they were disabled. So far, 3.6.15 is working without a hitch. I don't know why Firefox was hanging all the time, it was a new problem for me, I think it started after one of the automatic updates. No one has seemed to address this issue of the freezing (Not Responding.)

I suspect that Firefox is freezing up while it is busy sending collected information (cookies) back to Mozilla for use in pop up ads on web sites we go to. I do not have that problem when using Opera or a few other browsers. Opera is also much faster at bringing up a site since it is not busy sending info in the background.