Recently I experienced this again: after opening Firefox on my Android smartphone there was a notice about a new contribution in the “IRL podcast”.
Even though this is not a commercial ad but an offcial podcast by Mozilla (also see https://irlpodcast.org) it is still irritating when such notices appear.
To disable this you have to set the option
false in advanced configuration (about:config).
You have to restart Firefox before the change becomes active (for example by removing it in the list of recently used apps).