Thanks Schugy. Unfortunately it still doesn't work.
At first I tried creating a Launcher on my desktop with the following:
LD_LIBRARY_PATH=/usr/local/ff3compatlibs/lib /usr/local/firefox/firefoxbut it didn't work. The error I got was that it couldn't find the file or directory /usr/local/ff3compatlibs/lib even though it's there.
I then thought the solution might be with the firefox startup script, so I changed the line
moz_libdir=/usr/local/ff3compatlibs/libNeedless to say that didn't work. I got the same message as before that I have the wrong version of GTK+.
I then tried your suggestion
export LD_LIBRARY_PATH=/usr/local/ff3compatlibs/libbut nothing happened. I didn't get an error message either. Simply nothing happened.
What I can't understand is that when I got the wrong version error message why does firefox say I have version 2.8? In Synaptic I see only 2.12; there isn't a GTK+2.8 anywhere.