Have vim and/or neovim run in every HTML input present on a page. Don't use an external editor, do not require the user to install extra software besides vim/neovim and the plugin. Have the plugin integrate well with existing plugin ecosystem (e.g. tridactyl, vim-vixen...).
Although Chrome/Chromium compatibility is nice, it is not required.
It seems the only option if you don't want to have to use an external editor is to have Firefox become a neovim client.