I would hazard a guess that it’s more of a “devs only test against Chromium” problem than it is a Firefox problem. It’s a problem seen under WebKit-based browsers at times, too. Gecko and WebKit often behave differently and have different performance characteristics than Chromium/Blink does, but that’s often not accounted for at all. The extent of QA on non-Chromium browsers too often stops at “it technically runs”.
That's also probably part of the story but ultimately as an end user the fact is Firefox is a bad experience I'm suffering through only to not use Chrome. I can't force websites to patch whatever memory leak is causing Youtube tabs on FF to eat 5 GB of ram for example.
