There is a simple keyboard shortcut, if you have copied formatted text and want to paste it into the Firefox browser as unformatted text.
To paste the text as unformatted plain text into Firefox browser, press the following keys:
Ctrl + Shift + V
No extra plugins are required. This was tested in Firefox 52 on Windows 10. If this works in a different Firefox version or does not work in a specific Firefox version on various platforms let us and other users know in the comments section below.