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OK, much appreciated - many thanks for the time and trouble you've taken to explain all that. Looks like I may have to set aside an evening to go through it all!
I do have add-ons in use on one of my tabs, but used mainly to block those annoying ads on my inbox, and it seems to work pretty well, although where it used to remain clear of ads without any prompting from me, for some time I've had to click on the little arrow on the left side of the panel on the right of the e-mail screen, to get rid of that, whereupon the line ad at the top automatically disappears as well.
One more question: do you have any advice on how to install Flash on this computer? For the last couple of years, my access to TV and radio has gradually diminished - first, I lost C4, C5 and Dave, then ITV, and more recently I've annoyingly lost BBC, both TV and radio. They started putting a message on the play screen every time you clicked on a programme you wanted to watch or listen to, saying 'Sorry, you need Flash to play this programme'. But the programme would come up anyway, after a few seconds. But since April, that has not been the case, and apart from podcasts, and a small number of TV programmes, I can no longer get anything to play. I frequently resort to YouTube, where I have found a number of programmes uploaded, but it's not guaranteed, and programmes go missing from series.
Unfortunately, I have never been able to get Flash to load up to my computer. I have tried many times over the last 10 years, and followed all the instructions, but every time I just end up going round and round in circles, and getting nowhere. Recently, I tried a route that I'd not tried before, which told me to install Google Chrome, which has been a dormant tab since I first got this computer. But when I tried to do that, it told me that my computer was out of step with the time, and could not comply with my request until I reset it. Having tried and failed to do that, I got tangled up in a whole load of complicated instructions that meant very little to me, and eventually I just gave up.
Anyway, I will try the method you suggested, and give you the outcome later (don't hold your breath! I can be a bit slow with these things, being not very computer literate, and having been shown various things by the guys in the internet cafe - or just trying to puzzle things out by myself, with mixed results).
Thanks again.