I wonder how many of the people who say "Firefox is significantly slower than chrome" are using windows... On my computer, Firefox IS slower than chrome but (with ad blockers enabled) by an insignificant amount. By still being "the last remaining mostly independent, maintained and reasonably popular browser" I'd prefer it to use over chrome even if it is a bit slower.
Of course, ms is no longer the "old micro$oft" but their history on how they handle competitor browsers makes one think how much interest they could have in investigating and fixing such a bug.
My takeaway is: prefer independent software as much as you can.
I'm happy this was found and its not clear if this is already patched, but hopefully it will somewhat improve performance on youtube or other sites like it going forward.