This is nice, but all I really want is the ability to nicely see both English and German subtitles simultaneously. This is overkill for me and requires me to use chrome. Is anyone aware of a better solution for me?
(Obviously vlc and mpv will only work for DRM-free videos, e.g. from youtube or a DVD.)
If you were asking specifically about Netflix subtitles, there used to be an open source NflxMultiSubs extension[1] for both Firefox and Chromium, but it was broken by Netflix introducing changes to its video player and discontinued. There is an active, open-source dual-captions[2] extension, but it's for Chrome only. (However, since it's open source adapting it it for Firefox should be straightforward.) Finally, as an alternative approach, you could try a Firefox addon which allows loading arbitrary subtitles in the SRT format to netflix,[3] and which might perhaps allow you to have both netflix's subtitles and your own SRT ones at the same time.
[0] https://github.com/videolan/vlc/blob/master/NEWS#L30
[1] https://github.com/dannvix/NflxMultiSubs/
[2] https://github.com/mikesteele/dual-captions
[3] https://addons.mozilla.org/en-US/firefox/addon/netflix-srt-s...