Longtime user of Vim (needing to edit textareas) here.
After It's All Text! and Textern, I needed a replacement extension in Firefox to edit textareas in my favourite texteditor (Vim/Gvim). Found this neat page after searching for 'firefox extension to edit textarea externally inside browser'. Installed it using the instructions. Works like a charm out of the box. What's more, as described it works for any text field. It also works on discogs.com, where Textern didn't seem to work on the Notes textareafield.