I just tried opening the link in Mull and it is amazingly fast. It takes only 1 sec to render after zooming in or scrolling a lot at once. I'm a bit surprised because the file is 300+ pages. But I do encounter slow PDF loading sometimes, then I download the file and open it in another app
Does the webpage say anything about the extension being unavailable or something? I don't know if stable Firefox has that one enabled. I use Nightly which had allowed all extensions for a long time and idk about Stable