tasks: Update to Firefox nightly builds
tasks: Update to Firefox nightly builds

This will finally get us off the old version 69.
https://bugzilla.mozilla.org/show_bug.cgi?id=1593226 was fixed, so
versions ≥ 77 work again for testing Cockpit.

Unfortunately neither upstream beta nor Fedora downstream builds contain
the remote debugger any more, thus we are forced to use nightlies now.

Explicitly install dbus-glib and libXt, they are external dependencies.

cockpit-project/cockpituous,cockpit-project/cockpituous