OK, got to the bottom of the problem - it is down to some incompatibility with the NoScript extension (I failed to find that out earlier as I forgot to restart FF after disabling NoScript) 
With NoScript disabled, things on the WD2go site work normally
With NoScript enabled, (and WD2go.com is marked as a trusted site in NoScript’s preferences), I cannot log in. But NoScript does not indicate in its status line that it has blocked anything…
I have started working through NoScript’s settings, trying turning things on and off, but not yet found the thing responsible for my problem.
Regards,
Richard
