Please clarify in more detail. If in compose screen, this is normally straightforward. From inbox, it is not.

My Thunderbird is set up in split screen, snippets of the email on the left, and the body of the email I selected on the right. I wanted to copy an email address by highlighting it and right-clicking. But when I press and hold the left click button/double tap touchpad, I go to drag across the text. It just wants to slide the whole email as if I wanted to advance to the next one.

Seeing a screenshot may help, as I cannot visualize the screen you're seeing. When I use touchpad and highlight the FROM email address a drop-down menu appears with 'copy email address' as an option.

It's a mute issue now. After a few more hours of use, I realized Mozilla isn't the product that I was looking for, so I am giving an alternative a try.

It's a bug, one of https://mzl.la/3R5fiU9. And unlikely to be fixed soon.