You're here and "logged in" now. What's the question?

Could not get to "Help" for Thunderbird. Still can't. Prob.. When I copy and paste something from another email into one I'm composing, it changes the font for the rest of the email. Why? And how do I stop it?.

Try using Paste Without Formatting. Or ctrl+shift+v.

When you copy text from an environment where formatting is being used, it's hard to tell exactly how much of the surrounding hidden markup you're taking along with it. The text you think you have selected may include font markup, div, span, paragraph tags, line breaks and so forth. And it may include just the opening or closing part of a tag pair but not its partner. Under these circumstances, with unmatched singleton tags, it's not too surprising if imported text goes awry.

I get just these same problems even in expensive software such as Word; if a flagship product from the predominant software supplier can't get it right, then that tells us the issue is non-trivial to do right.

In Thunderbird, you can add in html code viewers that let you dissect out the text without tag fragments. Word doesn't have this capability and you just have to learn its quirks to control selection limits.