Checking it lets you view embedded images inline, rather than as attachments. Its recommended that you don't check View -> Display Attachments Inline. You can add it to the toolbar by right clicking on it, selecting Customize, and then dragging the button to it. If you prefer to display messages as plain text but occasionally need to view some as HTML the Allow HTML Temp extension lets you add a 'Show HTML' button to the toolbar that will display the selected message as Original HTML, and then revert to the View -> Message Body As setting when you select the next message. Its similar to the Original HTML setting but will only interpret basic HTML commands, disables Javascript, and doesn't display remote images.

The third option Simple HTML isn't used as much. This doesn't just interpret it as plain text, it temporarily gets rid of all of the HTML tags so that you don't see what looks like code. If you set it to Plain Text it will display a plain text message as is, and transparently convert a HTML message to a plain text message. This will display a HTML or a plain text message as is. View -> Message Body As defaults to Original HTML.

For example, when you reply to a message it may switch to whatever format is specified by 'prefers to receive messages formatted as' in the recipients entry in address book unless you set some other preferences. One problem is that Thunderbird's behavior when replying to a message depends upon what version you use. If you are uncertain whether a HTML message would be acceptable send a plain text message. However, some people strongly prefer to receive plain text messages. In most cases it's just a question of personal preference which format you use. HTML adds the ability to use additional formatting such as bold, italics, different fonts, tables and embedded images. At one time plain text messages were limited to 7-bit ASCII messages with plain text attachments but nowadays almost all email clients support MIME (for interoperability with Microsoft email clients etc.) so you can use other character sets, binary attachments and multi-part message bodies. You can create messages using either plain text or HTML. (Redirected from Plain text e-mail (Thunderbird))

Draft js get plain text