Textern is a Firefox add-on designed for Linux users that enhances text editing capabilities within the browser. It allows users to edit any textbox using their preferred editor, such as vim, emacs, or sublime. The functionality is initiated by selecting the textbox and pressing Ctrl+Shift+D, which opens the chosen editor, streamlining the editing process. Textern is built on the WebExtension API, ensuring compatibility with future Firefox updates beyond version 57.

To use Textern, users must install both the add-on and a separate native application due to the WebExtension API's limitations in launching external applications directly. The default editor is gedit, but users can customize it in the add-on preferences. As changes are made in the external editor and saved, the corresponding textbox in Firefox updates automatically, providing a seamless editing experience.