No, that is not possible. What kind of changes would you want to make ?

Because Firefox treats the PDF viewer as part of the browser, add-ons are not allowed to change it or add tools to it. If you have an add-on that you want to be able to modify the viewer or act on PDFs you view in Firefox, you can load the PDF into the web app version of the viewer which Firefox treats as just another web page. You'll notice an extra "open file" button on the toolbar of that version:

https://mozilla.github.io/pdf.js/web/viewer.html

Possibly there could be an add-on that automatically opens PDFs in that customizable viewer, but I haven't searched around for it.