This seems like it's mostly an extension similar to vimium, but you have to rebuild firefox to use it.

Edit: the build finished, typing from Glade. It is indeed mostly just an extension on top of firefox, but slightly better integrated, e.g. normal/insert modes in text areas work well. Doesn't seem to work with EDITOR=hx (maybe it doesn't know that it has to run it in terminal?). The integrated scripting engine is neat but I already have declaratively configured tampermonkey in my firefox. Oh, also, there are a couple bugs indeed.

For some reason I was expecting more, I don't know what exactly.