Henry wrote in comment 47:
there are no safety mechanisms in place when you use a formatting option or an insertion that will be lost
Actually, there are: It's the "Ask" dialog, which you are removing here. So, what you are really saying is that you should not remove this Ask dialog and preference option. So, given this argument, I'm actually going to say that the "Ask" dialog should not be removed.
I understand that you're talking about people who set the pref to "always plaintext". That value is for people who have seen the warning, and made their mind up, and do not use blue background and Helvetica 96px font. Users who set the preference to send 'only plaintext no matter what', they know full well not to use formatting. But they need the HTML composer for its other comfort, like quote handling.
I would consider the use of the HTML editor for sending only plain text an unsafe case that should not be exposed to the user.
The HTML composer is designed specifically for sending perfect plaintext. "HTML composer" is a actually mis-nomer, because the HTML composer is actually our primary plaintext composer. In fact, the HTML composer is better for sending plaintext than the plaintext editor, for reasons that are far far too complicated to explain in a single posting, but it would be a chapter of a book, or even an entire book to explain it all. We have a lot of code - all over the application and even in Gecko - to optimize this particular code path (HTML composer sending plaintext). Even all the way back to the mail viewer, so that the recipient can see the plaintext email perfectly, with wrapping line breaks, even within multiple nested quotes, and all that. Our plaintext handling works as designed only with the rich text composer.
This concerns wrapping line breaks, clearly designated and machine-readable quotes, nested quotes, and many other details in our plaintext handling that make email reliable. What's nice in Thunderbird is that the user doesn't need to worry about the details of formatting plaintext properly, like proper quotes, but we take care of creating the proper plaintext email, whereas the user uses a comfortable editor.
You've cited Outlook's behavior before. We are far superior to Outlook here. Our users specifically choose to not use Outlook, and use Thunderbird, in part exactly because our plain text handling is far superiour to Outlook. It's actually one of Thunderbird's core qualities. You're proposing to remove a core feature here that many users appreciate.
Thunderbird is popular as mail client, in part exactly because it has these options for sending perfect plaintext. Please leave these options for end users open, who want to use that.
You're touching settings here that are really core to Thunderbird, one of its main qualities, and options that many users feel extremely strongly about. HTML vs. plaintext is the classic flamewar in email. Taking choices from the users away is guaranteed to cause a huge backslash. And there is no need - these features and settings have proven to work very well.
I concur that the recipient-centric options can go away. I do not think that many users use them. However, I know that many of our biggest advocates and fans care deeply about the sender-specific options, particularly the mail.default_html_action, the "send plaintext when possible" settings, and their respective default values. Changing anything at all there will change all emails that a user sends out, will be noticed immediately, and will have a serious cost in losing mindshare and love from our users. You'd be enraging our userbase. I'm trying to say this the nicest way possible: Please don't do that.