How to keep "Add Yahoo mail" as an app from popping up in firefox?

That prompt is usually sent by the website as a web app/PWA install suggestion, not by Firefox as a normal notification. Firefox does not have a per-site “never offer to install this app” switch that always works for every site banner. A few practical checks: make sure Yahoo Mail is not already installed as a web app/shortcut, clear only Yahoo Mail site data/cookies once, then dismiss it again. If Yahoo is drawing the banner itself, an ad/content blocker rule may be the only way to hide that exact element. I would avoid disabling all notifications globally unless you are also getting separate permission prompts from the address bar.