In desparation I did a Reset of FF and it seems to have cured the problem. I don't like not knowing details, but a quick fix was welcome now.
FireFox Tools Options Advanced 'cashed web content' is now grey'd out. Automatic seems the standard setting, but I know I didn't alter that.
It still hangs at 36% in Ccleaner, but only for a few seconds. The list of Profiles is minute now by comparison.