Great idea! I was thinking about something similar too. Having multiple browsers doesn't have to suck...and having to fiddle with galternatives or KDE systemsettings isn't ideal.

May I suggest you to add a small (and optional!) config file to automate your UI - like e.g.:

    [Sources]
    Telegram=chrome profile B
    Discord=epiphany

    [Destinations]
    https://www.google.com=firefox profile A

If the config is there, you'd then only show the UI when neither sources nor destinations match (regexes). This would make it a must have tool IMHO :)