The MXKAppSetting properties httpLinkScheme and httpsLinkScheme allow for the use of a third-party browser to open links. This works great for Chrome but not for Firefox which expects a URL of the form firefox://open-url?url=encodedUrl where encodedUrl is the target url encoded as query param (this is done by addingPercentEncoding(withAllowedCharacters: urlQueryAllowed).

This can become quite complex (as is at Telegram) because some browser expect a format like dolphin://http://whatever.com or yandexbrowser-open-url://escapedUrl where escapedUrl is url-host escaped.

While I think, the latter stuff is unneeded, the support for Firefox would be nice and is also asked for at Riot. I will work on this.