Email is easy if you are using mutt. HN comments... I'd like to know that, too. Before Firefox switched to WebExtensions, there was a plugin that opened gvim when you pressed some hotkey while the focus was on an textarea. Now there is textern, but it requires some additional binary.
There still are plugins, you just need a helper tool on your system to run the actual commands.
Tridactyl provides the feature but if you just want external edit without the intrusive vim binding layer you can try Textern, it worked well back when I tried it.
