Vimperator is gonna break for good by the end of the year. It's been encouraging that the VimFX developer has been so involved in the Web Extension discussions about keyboard input [1]. Unfortunately, the last few updates don't look super promising [2].

1. https://bugzilla.mozilla.org/show_bug.cgi?id=1215061

2. https://github.com/akhodakivskiy/VimFx/issues/860