I really couldn't care less how bad the default editor (for textfields) in Firefox (or any browser for that matter) is, if it weren't for the IPython Notebook. If you are used to Vim, as I am, it sucks. Big Time.
Pterosaur embeds Vim in Firefox. Not Vim-like bindings, Vim!
It still has its occasional warts, but (most of them) are quickly fixed by its developer.
If you consider yourself a Vimmer and edit a lot of text in Firefox, I cannot recommend it more highly.
(And no, not every sentence in this post contains Vim)