For macOS 10.9, 10.10, 10.11 the most current Firefox you can run is the old Firefox 78.15.0esr

Firefox 79.0 to 115 ESR requires macOS 10.12, 10.13, 10.14 to run while Firefox 116.0 to current requires macOS 10.15 or later to run.

Unless your hardware allows you to update to at least macOS 10.12 to use Firefox 115 ESR.

You will still be able to use the old Firefox 78.15.0esr as well as you can now however your extensions may not work on March 14 on.

Thank you, James, for reply. V. helpful and confirming that I will need to buy a much later Mac desktop. But please would you explain what root certificate means and what the extensions mean as applied by Firefox. My concern is that somehow I may be cut off from the parliamentary systems I use every weekday in my employment with the House of Lords

Charles

Hi Charles

The certificate in question makes sure that add-ons that you have installed in Firefox will continue to work.

More broadly speaking, if your copy of Firefox is not supported (or coming to the end of support) due to the operating system not being supported (or not long off not being supported), from a security perspective, I strongly suggest that you look into a new laptop for work very swiftly.

Thank you, James, for reply. V. helpful and confirming that I will need to buy a much later Mac desktop. But please would you explain what root certificate means and what the extensions mean as applied by Firefox. My concern is that somehow I may be cut off from the parliamentary systems I use every weekday in my employment with the House of Lords Charles

This article explains it all though it is a bit of a read. https://support.mozilla.org/en-US/kb/root-certificate-expiration

The Firefox you have will likely still work the same with whatever sites you use it with currently. However many sites now days expect people to be using more current versions of Firefox for the site to work properly. You likely cannot view DRM content at the moment anyways.