That could be a Mint thing; I still open the "containing folder". I have to right-click the file to get a menu containing that option. EDIT: i.e. right-click the file name in the downloads list.

Thanks! Didn't occur to me to try a right-click.

I must be hallucinating - sorry but this doesn't solve the problem.

Right-clicking on the FF download dropdown right-hand column with the folder icons does produce a pop-up menu with one option "Open Containing Folder" however selecting this doesn't cause the folder to open, it just opens the image in the image viewer.

Is that still Firefox defining what to do with opening folders or applications or whatever?

An Arch user had the same issue almost a year ago: https://support.mozilla.org/el/questions/1298089

It's still working on my system.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I changed the file manager in Linux Mint from PCmanFM to Nemo and it now works, so, problem is something to do with Linux Mint and the file manager.