I suspect the issue is that the download UI that show-builtin-ui opens does not talk to the new downloads
API, i.e. is effectively broken in gecko >= 26.  It may be that if we want something similar we would
have to implement it ourselves or copy it from Firefox (possibly this would not be feasible depending on
how integrated Firefox's download manager is).  In that case the only solution would likely be just
removing the command.  Note that download-show does work, as sort of an alternative.