My guess is that it's something to do with Mixed content blocking in Firefox. This is when the website is HTTPS, but there's still content that's not HTTPS. Generally, when images are not HTTPS, there's no issue. But when there is a script (which is what makes the button function work), it usually gets blocked.

I'd recommend trying to disable mixed content blocking on the website to see if that makes a difference. See Mixed content blocking in Firefox for more information.

My other guess would be that it's caused by Enhanced Tracking Protection. See Enhanced Tracking Protection in Firefox for desktop for information on configuring the feature or disabling it on broken websites.

Hope this helps.

Works for me.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content. You can click shield icon for more detail and possibly disable the protection. You can check the Web Console for messages about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

Start Firefox in Safe 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 Safe Mode start window

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

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

Also make sure that the time on your computer is set correctly.

Thanks to all who have suggested actions.

I have tried so many things that I am not sure which action actually cured the problem.

I have an impression though that after having cleared cookies and cache for this site, the next time I accessed the site the usual warning window about accepting cookies popped-up. I usually respond to that kind of warning : "Accept all". This time I just dismissed the window, not responding at all. Is this (one of) the reason(s) the faulty button is now working ?

In any case my problem is solved. Thanks again very much