Yeah, that specific case wasn't fixed; it appears that Mozilla doesn't intend to let addons run on `about:`. However, they did make `about:` respect DNT by disabling GA.
WebExtensions currently fall short when it comes to any addon that modifies the UI significantly (tree-style tabs etc), though it's been mentioned in some other comments here that they're talking to addon devs to extend the API where possible.
I was curious how long it's been around, and did some digging. It seems to have been launched in 2009[1] and greatly expanded between 2011 and 2013[2].
Yes, the entirety of C is currently allocated to Canada[1]. The parent comments were talking about IATA codes, though, which aren't allocated by prefix.
I ended up writing a wrapper around youtube-dl and ffmpeg to download the MPEG-DASH video and audio streams and mux it because of this. It was a spur-of-the-moment hack, so don't expect error handling.