Came to say this. Vimperator cannot operate within constrained add-on sandboxes. Chromium already demonstrates how it works -- you get your vi keys on the page but as soon as you open a new tab and start entering your URL, nothing familiar works as Chromium takes ownership for the new tab.

I dabbled with DWB a few years back. It shows a lot of promise but it wasn't worth the hassle back then. If firefox is sure to go down, then all bets are off.

(Btw, dwb is deemed dying too, on the account of using webkit. uzbl is more maintained but uses webkit as well.)