Paste as Markdown от Medusis

Paste as Markdown.
Paste the content of the clipboard as Markdown, in any web form. The content of the clipboard can come from anywhere (a text editor, a graphics program, another web page, etc.) and be in rich text format with italics, bold, lists, etc., and it will be converted to Markdown upon pasting.

The clipboard content is unchanged, the conversion happens when pasting.

Everything happens within the browser, nothing is sent outside of the browser.

Some bugs in Firefox (two, in fact) make this extension somewhat fragile for this browser. However the latest version (0.5.7.3) is working in FF 80.0.1.