There's also Saka Key [ https://addons.mozilla.org/en-US/firefox/addon/saka-key/ ] that has an vim keybindings mode. It looks like currently Vimium-FF cannot open new tabs...
Personally, I'm running Firefox 55.0.3 with `vimium-ff` 1.6.0 (current build on addons.mozilla.org), and the `t` keybinding opens new tabs just like `Ctrl-t`. Command repetition also works (eg, `5t`).
reported: https://github.com/philc/vimium/issues/2510
closed with: https://github.com/philc/vimium/pull/2602
I'm not familiar with their release cycle, so I don't know when it'll hit AMO.