Well I've done all that, and I have deleted my bookmarks (backed up first) and uninstalled and reinstalled Firefox (it only happens on that browser). The result? No saved passwords and I continue to get adverts dropping down.
I guess I'll have to abandon Firefox and use Chrome or Safari instead. Pity, I found FF the most pleasant to use.
UPDATE. Finally killed it. I started deleting programs I recently downloaded. Found the culprit was smuggled inside FirstRowSports which probably serves me right as I shouldn't have used the site.