Earlier quoted context omitted.
Strange. I almost exclusively use OSX (El Cap) and haven't had performance issues asides from needing to restart the browser once a month or so. Hopefully they get those problems resolved soon, the browser is super quick when it's working right.
I find that I cannot watch a YouTube video on my MacBook in Firefox for more than 10 seconds without the fans coming on at full throttle. This is one of the few reasons that I start up chrome, as strangely I can watch the same video in chrome without performance issues. I haven't had this issue on Firefox on PC
If your cpu does not have a webm decoder in hardware, it has to decode them in software which is inefficient.