Works for me when I open the attachment in a new tab (e.g. via the contextmenu).

Do you have any other extensions or special configuration that may affect Gmail or PDFs?

And since you're mentioning PDF files: Firefox 78 has a new option in the download dialog to view PDF files directly in the browser. It looks like this: https://bugzilla.mozilla.org/show_bug.cgi?id=773942#c28

EDIT: To clarify, my comment about Firefox 78 is independent of your original review. If the extension doesn't work, open a bug report on Github or send me an email. I pointed to the upcoming new feature of Firefox, in case you only installed this extension to open PDF files directly in the browser (and no other file types). If that is the case, then you don't need the extension any more in Firefox 78+ because of the built-in functionality.