Five stars for legacy version 1.18 – tested OK with Waterfox Classic with the URLs below, which were problematic with alternative extensions:

https://github.com/progit/progit2/releases/download/2.1.109/progit.pdf

https://www.st.com/resource/en/datasheet/stusb4500.pdf

https://ec.europa.eu/newsroom/dae/document.cfm?doc_id=62684

I assume that version 2.9 for Firefox Quantum is equally good.

My previous review:

https://web.archive.org/web/20191209154542/https://addons.vaultpit.io/en-GB/firefox/addon/open-in-browser/reviews/902033/