First, many users write in text-only mode. They see email for communication, and since HTML is a markup language, you can never know how your message is received. And, all of what you want to do can be done in Thunderbird. Multiple fonts, different sizes, underlines, etc. But, since HTML is a markup language, it sometimes requires some tweaking. You can do some extra editing - and also see how the markup language is applied, by adding this addon - thunderbirdHTMLedit, from https://betterbird.eu/addons/index.html

How did this address the line wrap issue? And, I am aware of the nature of HTML (hypertext markup language) and of editing it - which seems to have become LESS supported in later versions of Thunderbird than in earlier ones - so many add-ons being disabled.

I TRIED to install your linked add-on from the downloaded file but, when I clicked on "install from file" T-Bird did exactly NOTHING - and I did try several times. I searched and found an add-on for HTML editing that suggested it was 'simpler' than thunderHTMLedit and installed that - since the search didn't list the former.

I would LIKE composition in T-Bird to be similar to that in a word processor. IS that quite impossible? I did ask "Doesn't Outlook do that' (that is, integrate with WP functions). But I've used Outlook so little and so long ago (at work). MS Exchange replaced the Novell Groupwise which I greatly preferred.

You never clarified whatever the linewrap issue is, so I never commented on that. MS Outlook desktop software may do what you want, as my perception is that the MS word processor function can create email (but it's been two decades since I've used that software). Otherwise, I am not aware of any email client that handles HTML as you desire. That is largely due to the dependence on HTML tags as a markup language. Use of Thunderbird for extensive HTML composition is a rare question here, as the majority seem content with occasional HTML emphasis facilities. I do recommend you try the thunderhtmedit addon again, as it is superior by far, in my opinion. And simpler to use.

To my mind, lines should automatically wrap with window re-sizing. This time, Thunderbird DID respond to the "Install from file," and I installed the thunderHTMLedit.xpi (v. 3.4.0).

Elsewhere, I submitted a suggestion that Thunderbird development consider partnering with LibreOffice to extend composition capabilities. Though LibreOffice can read HTML files, I'm sure it isn't using HTML for formatting.

Is there any reason it would be necessary to use HTML?

HTML is needed because messages are transmitted in simple mail transfer protocol - plain text. The styling aspects must be additional codes to instruct the receiving email client how to reformat and present the intended result. Iibreoffice has no such restriction and can use the full character set to prepare the final document.