A little update: this works if you keep TFF prefs useragent to FF45. If you change it for something else, the global setting precedes the override key!
This might explain why it didn't work when I first tried this trick on Youtube, as my global useragent was set to FF52 by that time. :-)
Yes, that's intentional, since the global setting is the only one exposed in the UI. This way it's obvious why something is occurring. If/when other options are exposed in the UI, then I'll reconsider this.