Vimperator was abandoned when XUL add-ons were abandoned, presumably because they couldn't pull it off with WebExtensions.
Partially, the reasoning they gave back then was it was too tightly integrated with Firefox's internals. Even if WebExtensions had all the APIs needed, it would have needed nearly a complete rewrite and they weren't up to it.
As far as keyboard browsing, doesn't vimperator cover that?