Accessed my Ebay account first thing using Firefox on my Win7 laptop. No problems.
Later in the day tried to on this Win7 desktop. Firefox wouldn't run and threw up an error message. Quitted and trying again - same thing. Re-booted the machine - same thing. This machine has both Win7 and XP as separate OS - even on different HDs - so switched off, and then loaded XP, only to find the same thing with FF.
Went out shopping, and it seems back to normal now. Only common thing is they all use FF sync.