autofill for recipients' email addresses not working when I try to compose an email. Problem confined to Firefox - no such problems with other browsers.

Show us something. What site? Do you have the same issue with Firefox in Troubleshoot mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

The problem is that when I try to use autofill for an email address I get the message 'invalid field value.' This problem does not occur when I use Google Chrome so it must be something to do with Mozilla Firefox. I have tried all the suggested remedies in the Firefox help section.

I understand the issue, but I'm asking you to show the issue and what site?

Not sure I understand what you want. Typically I go into Mozilla Firefox, click on <bt.com> and then go into <email>. When I arrive at <Inbox>, I click on <compose>, I enter a partial email addres for the recipient I want to send mail to. After the first few letters the whole of the email address comes up autofilled in a separate window. However, when I try to get that full address to move to the main recipient window (by clicking on it or by pressing the space bar), the partial address fails to turn into the full address, gets a cross beside it and turns pink; and when I move the cursor over it I get the message 'invalid field value.'

By way of example I enter 'jack' for <jackbov.yahoo.com> but don't get beyond 'jack.' This means I have to type out the full address because autofill is not working as it should.

Does this give you the information you need?

I see the issue. see screenshot Shouldn't that be jackbov@yahoo.com and not jackbov.yahoo.com? Looks like you're missing the @ which is why screenshots are sooooooooo helpful.