macOS 10.13.6 (High Sierra) on a MacBook Pro is quite an older operating system.

You may want to use the Firefox ESR version.

You can also try this: 1. Type about:config in Firefox's address bar and press Enter 2. Click through the warning prompt 3. In the search bar on that page, type: dom.event.clipboardevents.enabled 4. If this value shows as "false", click the toggle button to change it to "true"

Good luck.

I now find it's possible to copy and past one line at a time, and sometimes a few lines at a time! How crazy is that?

Thank you Wxie. Sorry, before seeing your comments I posted again: "I now find it's possible to copy and past one line at a time, and sometimes a few lines at a time! How crazy is that?" Does this sound like something your advice would fix? I'm obviously hesitant to get involved in things I know nothing about like coding and Terminal etc.

Or you can start trouble shooting mode and try it. You can always change it back to it original value.

https://support.mozilla.org/en-US/kb/reset-preferences-fix-problems