Review for Outgoing Message Format by Geremia10
Rated 5 out of 5 stars
Even in TB 52.x, this extension is needed to enable default plaintext delivery formatting (so you can compose as HTML and convert to plaintext for delivery).
"prefer sending plain-text e-mails" will only deliver as plaintext if no HTML is detected during the email's composition.
To create your own collections, you must have a Mozilla Add-ons account.