SillBill Made no difference still won't start. I found nothing wrong with 18.0 and have reverted to it. I will let it settle for a bit and try again in a few days (weeks?).
thanks for your advice.
I am not going to mark the question answered yet.
Since the "solution" to the above post, I have had trouble with startup of my machine (W7). Often I am told there is a problem and windows will try to repair it - and does but I find firefox 18.0 installed instead of the 18.01 which I was running successfully before. Anyone else found this? I have decided to stay with 18.0 in the meantime.
No Norton. I am using MS Security Essentials which I have done for some time.
I had another go with 18.01 today and it is still OK but what will happen when I S/D and restart? We shall see.
I have just noticed that in the Firefox programme a number so folder names are almost identical e.g. "defaults" and "defaults(2)" "modules" & "modules(2)" etc. but others are not e.g. "greprefs(2)" and "chrome(2)" - no "greyprefs" or "chrome". The unmarked folders are the newest, all dated today, and those marked (2) are dated 1/12/12 (when I may have up dated but I can't be sure). Also, the folders have different contents; generally the newer folders contain many fewer files.
Can this be of significance and can I safely delete the older folders or should I delete everything and start from scratch?