on Thunderbird for Android the push connection and the new mail alerts are separate settings, which catches a lot of people. check Manage folders > Inbox and make sure both Enable Push and Notifications are on, then Settings > your account > Notifications > New mail notifications. if it still only turns up when you open the app, set its battery usage to Unrestricted, and on Android 13+ allow “Alarms & reminders” for it (that’s in the app’s system settings, not under permissions). FairEmail is worth a try too if you get fed up with it.

the affiliate theory sounds right to me, the hijack only pays if every search goes through their tagged link. for locking it down, regular Firefox supports enterprise policies. on Bluefin it’s the system Flatpak, so put a policies.json in /var/lib/flatpak/extension/org.mozilla.firefox.systemconfig/x86_64/stable/policies/ with ExtensionSettings set to "*": {"installation_mode": "blocked"} and "uBlock0@raymondhill.net": {"installation_mode": "force_installed"}. that removes the junk, blocks new installs and he can’t uninstall uBO again. restart Firefox and check about:policies to see it applied.