RTF Viewer от Tom Zöhner

This extension allows to view RTF (Rich Text Format) files in the browser and does so automatically when clicking any link to an RTF file. The RTF file is then automatically displayed in the same tab (similarly to the pdf preview feature).

This extension does not depend on any external services to generate the preview.

To open local files click on the extension icon in the toolbar, files can't be opened directly from the file system since this is currently not supported by firefox for security reasons (see Bug 1266960).