Are you certain you're talking about Thunderbird?

What are 'https videos'?

And what are 'www. attachments'?

Yes I'm using Thunderbird. https videos are videos that people send from the internet as attachments , the https. refers to the location site of those videos. www. are the same as above only a different location.

Still doesn't make sense to me. Attachments are files attached to a message. But there are no https files, https is a protocol.
So I guess what you're talking about are links to https web sites.
Clicking a link is supposed to open the corresponding web site in a browser.

... the sound comes through but no picture.

So are you trying to view videos in a browser, and the browser is what's giving you problems?

Possibly but on two other computers the same videos played with no problems. Something has changed on my computer. If this is the problem, how would I fix it?

If your browser is Firefox, try to ask in the Firefox support forum.