This when I try and click on a pdf file. Zilch.

What security software do you have?

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Thanks for your suggestion.

I have up-to-date ESET for security. Chrome does these downloads OK. Nothing that I know of has changed on my system. Firefox is version 79.0, 64 bit. I have tried closing Firefox and reopening it, no joy.

Right now, after having typed the above, I have just looked at my Firefox options and have found that pdf files (the subjecst of this topic) have (all of them) been set to be saved into a particular folder!! I have altered this setting to have the files opened with Foxit Reader (which is what I use for pdfs) and i now get the little "What should Firefox do with this file" dialogue box. I choose "open it" and tick the box for it do do this always in future, but the tick does not get remembered and before each file I can read, I have to answer the box, in spite of the tick being set. So I'm better off than when I started but still wishing that tick would be remembered. Suggestions welcome. Jon