Firefox 78 was the last version for your OS (Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release).

Firefox has moved on, of course, and is up to version 133, but I don't recommend trying to install anything newer than you have.

Is a website telling you the older Firefox is no longer compatible? Where are you having a problem?

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

If you have any questions related to Firefox on Mac (macOS) in the future, then you should post in the Firefox section https://support.mozilla.org/en-US/questions/new/firefox using this form https://support.mozilla.org/en-US/questions/new/firefox/form

I would move this thread from Firefox for iOS (for iPhone/iPad) to the desktop Firefox section as you have macOS but an Agent already processed this so I cannot move this thread now due to recent changes.