Try this add-on or maybe switch to a different reader.

Thanks for the suggestion of an alternate reader. I am a bit puzzled why the alternate add-on suggestion is from the Mozilla team who are responsible for the newly integrated reader. Why don't they leverage their own code?

More to the point, I wanted to get information to the developers so they can improve their code. Sorry if this is the wrong channel, but there was no obvious way to report a performance defect.

The add-on is from Mozilla themselves - it fixes issues present in the reader that can't but pushed out to the release. The built in reader (PDF.js) is fairly new and Mozilla is aware of the underlying issues. You can always use the Suggestion Box to submit feedback. Switching to an alternate reader may solve your issue - that's why it was prescribed.

OK, I tried the updated PDF.js that is provided as an an add-on. Unfortunately it exhibits the same performance problem as the built-in reader, so something is still wrong.

There is some improvement as it no longer reports an error and after a long delay getting to the first page, remaining pages do begin to display a bit more quickly.

Here are instructions on changing the reader so you can still continue to use Firefox.

In order to change the default reader for PDF files (to not open PDF files with Firefox's internal PDF reader), follow these steps:

1. Go to Tools > Options (or Firefox > Options).
2. In the Options window, select the Applications tab.
3. In the Search field, type PDF. You should find Portable Document Format (PDF).
4. On the right handside you should find an Action column. Use that to select your favorite PDF reader. In order to view PDF files in Firefox, choose Preview in Firefox.