I nuked the hard dive and did a clean reinstall of of Win XP SP3, Firefox 39 and Norton 360. Because my CPU chip, AthlonXP 3000+, does not support the SSE2 instruction set, I have to use alternate versions of Norton 360 and Toolbar. Win XP is completely updated and I ran Norton Live Update until it repeated that everything was current. At this point Norton Toolbar doesn't even show up in the Firefox Add-on list but it works fine in IE 8.
I get the impression there is some kind of bad blood between Firefox and Norton that has resulted in a lot of finger pointing and waiting for the other guy to own the problems of compatibility. At this point I'm throwing in the towel and moving on to more pressing problems.