This popup message appears if you are using the ‘Per-Site’ zoom mode and some of Firefox’s “about:config” preferences have been changed, specifically:

‒ if the ‘browser.zoom.siteSpecific’ preference has been set to ‘false’; or
‒ if the ‘privacy.resistFingerprinting’ preference has been set to ‘true’.

In either case, Firefox’s site-specific zoom functionality is disabled.