Firefox hangs when opening PDF files?

The best thing you can do is install the PDF download extension for Firefox.

Once the PDF Download extension has been installed, you’ll be provided some options whenever you click on a link to a PDF file. With PDF download, you can just download the PDF file directly, without having to open it in Firefox, thereby lessening the chance of Firefox hanging.

Additionally, you can view the PDF as plain HTML using the PDF download extension.

So, if Firefox crashes when opening PDF files, it’s time to give PDF download a try.