OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOK!
This was a long story. I tried disabling WebGoBoard. Woops! FF crashed anyway. WTF?
There was no entry for 'sgf' in mimeTypes.rdf in my FF profile so I checked the windows registry. In the registry sgf is assigned to Gowrite 2 (weird because if I double click on an sgf file, it opens in Drago not GW2!). I uninstalled GW2 and meanwhile used CCleaner to 'fix' all the issues in my registry. Reboot, restart FF and I got the text version of the sgf file. Reenabled WebGoBoard and suddenly it all works! I am sorry to take up so much of your time on what turned out to be my issue. I thank you very much for your patience and the key hint that pointed me to a solution. Thanks again for WebGoBoard!!!
(Cough, cough) BTW, is there a way to display coordinates in WGB?

Edit: My post looked strange after I posted it so I went back to the registry. There are various other registry entries for sgf files since I have a variety of programs on my PC. In any case uninstalling GW2 seems to have been what did it for me. However, it may have been some other registry fix. Then again it may have been something else...

Dave Sigaty
"Short-lived are both the praiser and the praised, and rememberer and the remembered..."
- Marcus Aurelius; Meditations, VIII 21