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.

The extension works just as I expected. I decided to install the messaging host manually for security reasons, but it was quite simple, and the extension works like a charm. I've been looking for this for a long time!