Is this about the web content in that tab or also in the user interface (e.g. Tab bar labels) ? Can you attach a screenshot? *https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem *use a compressed image type like PNG or JPG to save the screenshot Websites like Google and Facebook and YouTube have enabled Brotli (br) encoding for files send via a secure connection. Some (security) software intercepts a secure connection and acts like a man-in-the-middle to scan content and may not recognize this encoding and changes the content-type header to text/plain. This causes Firefox to display compressed content as gibberish instead of rendering the decompressed content. You can check the issuer of the certificate to see what software is interfering with the connection and places itself as a "man-in-the-middle" between Firefox and the web server. Make sure to have the latest updates of this software and possibly contact them for support.