That would involve upgrading your macOS version, see similar: https://support.mozilla.org/questions/1582867

Firefox 115.36.0esr released May 19 is the current update for the older Firefox 115 ESR. https://www.firefox.com/firefox/115.36.0/releasenotes/

Firefox 115.39.0esr out on August 18 is the last planned update for the old Firefox 115 ESR unless Mozilla decides before then to extends updates yet again.

Thank you James for your feedback.Unfortunately, it didn't work; I got the same response. Regards Humphrey

To understand why you're only getting updates on an older version, start here:

•  Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release

To upgrade past your 115.x (as the current version is already 150.x for supported systems), you'll need to upgrade your Mac first — might be just a matter of moving from High Sierra to Catalina if your device allows, but if that upgrade is not supported the options get rather complicated.