Vimium-FF was among the first Vim extensions for Firefox post-XUL, and is by far the most popular. It's from the same author (and has the same codebase / github

Vimium-FF was among the first Vim extensions for Firefox post-XUL, and is by far the most popular. It's from the same author (and has the same codebase / github repository) as the Vimium extension originally released on Chrome. Vimium C was forked off of that in 2014 and seems to be targeted towards Chinese users, with added support for the Chinese language. The Github readme does hint towards performance improvements, but doesn't provide much more detail. It also seems to have a new tab override feature, and a vimium-compatible fork of the Firefox PDF viewer, and a few other miscellaneous features.