I consider that a feature. If I go to youtube.com with firefox, it means that I intentionally want to avoid opening the app, so I don't want every video I click on in firefox to open with the youtube app.
However, I think there was an option to have it enabled if you wish to.
+1. YouTube example is spot-on, as this is no.1 case where I actively avoid the app in favor of the browser. YoutTube's app and mobile website have some ridiculously stupid extra filtering that silently removes many videos - videos that are suddenly accessible if you check the "Request desktop version" box in the browsers. It's almost an universal occurrence that the video I'm searching for is one that gets silently filtered out in the app.
However, I think there was an option to have it enabled if you wish to.