Overall, this is a great app. I hope it could be more robust than Vimperator as I have scrolled up the configuration files.
Here are a two things I came about in the first 20 minutes of usage:
1. the pasting in insert mode would trigger a different pass-key mode, which is a bit unusual. I mean, in insert mode, at least people should be given the right to past without further disruption.
2. That the tabs overhead cannot be clicked if one move to the up-most of the screen. Only after moving the cursor down a few millimeters would make the tabs clickable.
Lastly, happy Vimming in Firefox! ^.^