Does this only happen with PDFs you download through Firefox?

• If so, that would probably be some setting I don't know about...hopefully other can provide input.

However, if this happens for all PDF files, regardless of where they come from, you will want to check the default application selected by your operating system.

• If you are using Windows, have you checked the default application selected in Windows? Navigate to "Settings" (the gear icon in the start menu) > "Apps" > "Default apps" (in the left navigation bar) > "Choose default apps by file type" (link at the bottom of this settings page). Scroll down the .pdf, and slect the reader you want, if it is not already selected.

Thank you for your response.

I found the cause. There is a Firefox setting: settings/general/applications (you have to scroll down) and PDF had been set to “open in Firefox” I reset it to my usual PDF reader and all was fine.

A recent Firefox upgrade must have changed the setting (naughty, especially without asking), it also overrides the Windows default for that file-type, which was still set to the default (first thing I checked).

Ah, I had forgotten about that setting. Good find!

Strange that it got changed without asking.