I migrated from Microsoft Outlook to Mozilla Thunderbird due to license issues.
Generally, I find Thunderbird usable, but two things annoy me quite a lot.
One thing is that I cannot convert a new message from plain text to HTML (or vice versa).
I send 95% of my mails in plain text, so I definitely want plain text as the default format when I press Ctrl+N or click on a mailto: link.
But in 5% of all cases, I want to include an embedded image in my message.
So if I have created a new message using Ctrl+N and have spent effort typing recipient address(es) (and sometimes CC and/or BCC addresses) and subject and maybe even some introductory body text and realise that I need to include an image, it makes my sad that I need to discard all my changes and start all over again (possibly copying each text field to clipboard first).
The same thing applies if I have followed a mailto: link that includes recipient address, subject, and body text.
In Microsoft Outlook and most other mail apps (desktop or web-based) I have used, you can switch between plain text and HTML using a single toolbar (ribbon) button or menu item.
But apparently not in Thunderbird.