Is MacOS Big Sur 11.7.11 supported by Mozilla FireFox?

Hey Neil, yes it is supported. Actually Firefox supports all Mac OS versions from 10.15 or higher, you can read more about it here: https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

That KB article is more for macOS users using the EOL macOS 10.12, 10.13, 10.14 though.

The desktop Firefox since 116.0 and later including the Firefox 153.0.3 Release still only requires macOS 10.15 (ten.fifteen) at minimum to run. https://www.firefox.com/firefox/153.0.3/system-requirements/