What is the error screenshot saying? And what OS and FF version are you referring to here.

Hi, your Firefox said it is version 78 running on Mac OS X 10.11. If that is your correct Mac OS version, then you have the latest Firefox available for your OS. '''''Have you considered upgrading your Mac?''''' If you can install macOS 10.12 "Sierra" or later, then you can use the Extended Support Release of Firefox 115, which is still supported with security updates for at least a few month months. I don't know what will work on your hardware, but if you visit the Mac App Store in Safari, you might be able to find a newer version that will work. ---- You probably noticed a problem with any add-ons you run or want to install: [[Update Firefox to prevent add-ons issues from root certificate expiration]].