How to merge features with browser editor

I can not figure out how to merge two line features (streams) in the browser (firefox) editor mode. I read this old advice here How can I merge two paths in the iD editor? - OSM Help which says to select both with shift and a + icon should appear. I can select multiple connected features with shift, but there is no obvious merge button appearing.

You are probably trying to connect lines with conflicting data, which when combined will generate conflict, e.g. in relationships. Select these lines and open the context menu with the right mouse button - hover over the ‘+’ icon and check whether it is grayed out and if so, you should see a message describing the problem.