Only Spypbot S&D still seems to work on Win9x (lucky I used to have several) - it produced a raft of crud accumulated over time - but not anything related to this - and the problems still exist.

It seems that I have suffered bit rot of many of the FF system JS & some css files.

Discovered that the FF updates assume that the Mozilla App_data lives in the standard Win directory sub tree - not on another partition as i had it set up. Un & Re installs had failed, wonder if that could be the problem - going to try a new install with the default set up. Can't find
any Registry entry that would point there though.