Firefox

Unfortunately, Firefox 78 is the last version for Mac OS 10.9-10.11. You could try using an add-on that lies to the website about your browser version and see whether it works. But this won't upgrade the secure connection features, if that is the problem. For example: https://addons.mozilla.org/firefox/addon/user-agent-string-switcher/ Since it can cause connection problems on some networks, you may need to switch it on when needed and off the rest of the time. Plan C would be to use a more up-to-date mobile device.