Creator here.. there's netflixmultisubs extension which is more minimal, which we also maintain now. There used to be releases for firefox, but we haven't been publishing them, just becuase we're not familiar with the process and lack time. LLN can be used in a minimal, non-instrusive way however, and has a pretty good algorithm for aligning the start and finish times of different subs tracks. netflixmultisubs shows the two tracks with their independant timings.