Shaving a few microseconds off of internal loading is absolutely meaningless when there are hundreds of milliseconds of third party JavaScript being downloaded, and your browser is actively removong the features that allow you to not do that.
You'd be surprised. I had to chase down a rendering bug that made an infinite scrolling widget unusable on Firefox because it did recursive layout when flexboxes were within a scrollable div. A 2% performance hit can matter in the modern context of hundreds to thousands of layout elements.
That having been said... This is an ecosystem thing and if everybody were to switch to Firefox it would be Chrome that had the weird rendering bugs because people would be testing on Firefox first and wouldn't be using layouts that Firefox didn't handle cleanly.
