A few months ago, Thunderbird decided that it was going to ignore the fact that I’d configured it to always send plain-text messages, so that although new messages are plain-text, when I forward or reply to messages (whether HTML or plain-text) it always creates them as HTML.
I know I can stop this as a one-off by holding down the key before clicking the reply or forward button, but that’s not really a great solution.
Perhaps unsurprisingly, all of my searches on this subject come up with answers as to how people can get Thunderbird to reply/forward with HTML.
I wonder if any Manjarans have found a solution?
It used to be that you could change the prefers to receive messages formatted as in a recipients address book entry.
Then, that one recipient would receive replies according to whichever formatting you configured. I don’t have access to Thunderbird on this machine, but I found the following link that at least confirms this was the case (3rd paragraph).
Hope this helps.
Cheers.
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Hamburger menu → Settings → Composition → just below the “HTML Style” setting is:
“Sending Format”
… four options, one of which is: “Only Plain Text” 
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It looks like the OP already has this setting configured; according to their post:
I mentioned the setting in an address book entry above to override the mail sending format for individual recipients, but that option doesn’t seem to exist any more, in recent Thunderbird incarnations.
All I can suggest is to double-check the mail sending format settings:
- Automatic
If no styling is used in the message, send Plain Text. Otherwise, send HTML with a Plain Text fallback.
- Both HTML and Plain Text
The recipient’s email application will determine which version to show.
- Only HTML
Some recipients may not be able to read the message without a Plain Text fallback.
- Only Plain Text
Some styling will be converted into a plain alternative, whilst other composition features will be disabled.
- Perhaps a recent Thunderbird update has reset the default selection.
- Perhaps a Thunderbird addon is interfering with this choice.
If Only Plain Text remains as the selected default, then this could be a legitimate Thunderbird bug causing this setting not to be retained.
However, I have not found an issue or bug report that supports this.
It could be that manual selection of the format each time you reply to an email message is the only available option now.
I think you may be right. Time to raise a bug, I think.
Thanks.
If you did raise a bug with them, I’m unable to find it. Please link to it here.
@beermad
Can you confirm that your recipients don’t actually receive text-only email from you when you have Only Plain Text configured?
Or is it that you don’t see any difference in the preview before you reply? It might be that it works as expected even though your preview shows as HTML.
Just a random thought…
Cheers.
An interesting thought, thanks.
Though bizarrely, I can’t test this as for some reason it’s reverted to the “proper” behaviour. Either something I did in response to a suggestion here worked (perhaps it needed a restart) or the recent TB update fixed this, though I couldn’t guess which.
Apologies. I’d been too snowed under to find time to do that. Bizarrely, it’s actually stopped replying in HTML. Whether that’s due to something I did suggested here (perhaps needing a restart) or the recent update fixing it is impossible to tell.
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It would be easy enough. Contact once of your recipients and ask them what format they received an email from you (in the period that you had the issue).
Nonetheless, it’s fortunate you didn’t report it as a bug.
(My guess is that you might not have rebooted after an update that included Thunderbird. A common scenario 
).
Cheers.