I use qutebrowser, and one keystroke opens vim. When I close the editor the contents of the Vim buffer appear in the browser text-entry input.
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.
For email mutt is obvious, but I also recommend aerc, though it's still beta and does crash sometimes. And I'm writing this from w3m, in which you can just navigate to a textarea and write to it in your $EDITOR. For general browsing it's not that great due to its lack of js/css support, but I find it great for commenting on forums which usually still work with html forms.
Interestingly, w3m used to work with gmail as well, but sadly you can no longer log in without javascript.