Type about:preferences#applications<enter> in the address bar. In the search box, type pdf At the far right, press the box and choose.

Hi FredMcD, That just allows me to choose the default programme to open PDFs. It doesn't relate to why they open automatically on certain pages. Thanks

It might be the way the link is set up. Post a few below so we can check.

It's an intermittent problem, depending on the page. At the moment I can only find one http://www.theaustralian.com.au/news/nation/malcolm-turnbull-tony-abbott-promises-no-wrecking-no-sniping/story-e6frg6nf-1227529241841

Oddly, it doesn't happen from my wife's laptop. It must be my synced Firefox setup.

Well, I hit a paywall there.

When you're on such a page, if you press Ctrl+u to load the source viewer, does anything come up? If so, it's probably a PDF embedded in the page instead of being displayed "stand-alone" (in which case the download setting should apply).

I found the answer on another forum. It's to set the PDF preference to preview in Firefox, as explained here - https://credoreference.zendesk.com/hc/en-us/articles/201757169-Firefox-Automatically-Downloads-Opens-PDFs

Cheers!

That was very good work. Well Done.