Rated 5 out of 5 stars
I've tried a number of different add-ons over the years to get keyboard shortcuts for switching between tabs. The best I'd come across was SwiftTabs, but this is no longer being developed, and isn't e10s/multiprocess-compatible.
Two things in particular make VimFx ideal for my purposes:
* it doesn't change anything about Firefox apart from keyboard shortcuts (unlike other shortcut-related add-ons, such as Advanced Tab Navigation, which also have the side effect of altering things like the placement of new tabs, even if you turn off all their options)
* it is highly customisable -- for example, I don't actually want vim-like shortcuts (much as I'm happy with them in vim itself, I prefer other shortcuts in other software :-)), and so I can configure VimFx to use exactly the kinds of shortcuts I want in the way I want to use them (e.g with '<force>' so text input focus doesn't get in the way)
Bonus points too for detailed and useful documentation on the GitHub site, which makes it easy to understand exactly how to set it up the way I want it to be. At the moment, I'm only using VimFx for tab-related shortcuts, but it has such a comprehensive range of options that I could certainly see myself gradually making use of it in other areas in the future.