I had what appeared to be a similar problem.  I was able to keep Firefox from
hanging, but had to select the "work offline" button in the "file" menu.  The
site would display incomplete.

Looking at the html code of the displayed file I found that some of the links
and image pointers did not resolve correctly and still pointed to the Internet
locations.

Sometimes the locations can be fixed by running the mirror one or two levels
deeper than necessary.  You'll have to fix them manually if this does not
work.  I also mirror a site which uses a javascript file - HTTrack does not do
well with most javascript and I have to rewrite parts of that file every time
I run the mirror (or use an old version, if I can determine that it is the
same).

I hope that this at least explains what's happening to you, though I might not
have the same problem as you.