I checked the repo, there was a slightly later version, but installing that made no difference. After a bit more experimentation that had no effect, I renamed my personal .mozilla folder to put it out of use, opened FF again and found that the BBC iPlayer was OK. But I'd lost my personal settings, so I began copying back the folders from the old .mozilla/firefox/random-number.default file into the new one that FF had created in blocks in a systematic way to try to locate the faulty folder/file. Luckily it didn't take too long as it was right at the top of the list - the Cache folder. I put back all the other files and folders except, of course, the Cache folder and everything is fine, all my settings are there, and iPlayer works.