It's unlikely they put any effort into intentionally make them run slower, it's just that they are written to work optimally on Chrome and minor differences in the behavior of things like the V8 vs. SpiderMonkey and Blink vs Gecko. Given that each one is written with different tradeoffs, it's not surprising things perform differently.
Whether or not the Google programmers use specific proprietary knowledge about the behavior of Chrome to optimize performance is different. If they do, that would be similar to the things that got Microsoft in trouble.
Google knows that every time they release a Firefox bug, FF's user percentage goes down a tiny bit. Repeat over dozens of bugs, for years, and you have a strategy.
There's one blog post from another Mozillian that I can't find anywhere that came out within the last year with other examples, I think it was on HN.
