Great plugin for emulating Vim key bindings, with good default bindings that work well on most websites. The bindings, behaviour, and appearance are all fully customisable, to the point that you can replicate the behaviour of many other add-ons with a couple of lines in your tridactylrc file. The current beta version now includes tab groups. The biggest drawback is that Tridactyl doesn't work on PDFs or special Firefox pages - unfortunately, this is due to FF's security features, and is true of all similar add-ons, too. The escape hatch (Ctrl+,) helps mitigate this so your flow isn't completely interrupted.