When you open an attachment the first time, a window appears that asks to set the opening app, and there is a check box to perform the same action every time an attachment is opened. Checking the box stores the setting in the Files & Attachments section as in your picture. There is a bug that prevents the setting from being stored: https://support.mozilla.org/en-US/questions/1353036 But you should have an entry for pdf files that selects the built-in pdf viewer, as this feature was introduced in TB 91. If you're still on an earlier version, the bug isn't present, and the setting should be stored.