Try this: Open the TB profile: - TB menu > Help > Troubleshooting information - under the Application Basics section next to Profile Folder, click 'Open folder' - close TB > rename the handlers.json file in your profile folder - restart TB > go to TB menu > Help > Troubleshooting Information > click the 'Clear startup cache' button After restarting, when you try to open one of those attachments, you should be presented with a screen where you can choose which app to use. If that didn't work, try this: Open the config editor: - TB menu > Settings > General > scroll all the way down > click the 'Config editor' button Search for and set these to true: - network.protocol-handler.warn-external.http - network.protocol-handler.warn-external.https Then try to open one of those attachments. If it works, set the two options back to false.