Description of the reporter on https://webcompat.com/issues/18352 ---------- This site is the mobile page of bilibili (a large video website in china). In fact, using the Firefox for Android to open any video link on this site (bilibili.com), you will see a hint that the protocol is not available. The same error can be seen by using the responsive mode of Firefox desktop version. Using other browsers will not. From the request point of view, it seems that the website has a request to use the native app (bilibili://).Other browsers will ignore it while Firefox will load the request. https://webcompat.com/uploads/2018/8/8cc4dc7e-8f69-439b-b3f7-d7005f1369e2.jpeg ----------

We still wanted to show the error page for URLs manually entered by the user through the browser chrome. Unfortunately the protocol handling code currently doesn't have a good way of finding out who caused the URL load, so as a workaround we suppress the error page if the load happened within one second of some user input within the web content (bug 1278581). This handles cases where the user directly clicks on some link with an unknown protocol, but of course doesn't work if the page itself tries to open an unknown protocol without any user interaction.

Also it might be better if instead of using a custom protocol directly, that page used an intent://-URI [1] instead, since that way they could also specify a fallback URL which we would always honour. [1] https://developer.chrome.com/multidevice/android/intents