Currently I always copy the URL from browser to browser to browser for testing.

Not required to auto detect everything. A preferences box to add new external applications
with command line would be fine. And then a Combobox in the toolbar, plus a new menu "Browser"
for fast switching of browsers would be fine. Add keyboard shortcut support for the new menu
and we could switch browsers quickly with CTRL+F1 to CTRL+F9 for example.