Rather embarrassing that they didn't notice this in testing, but it only affects the homepage and not searches from the address bar
They probably did. However wanted to screw Firefox users the same way as the YouTube slowdown.
Not saying that they shouldn't have detected this (i.e.: where is the automated testing for those things?), but I don't think this is Google screwing up Firefox on purpose.
And if Google really wanted to screw up Firefox, they would probably do a better job than User-Agent sniffing. Firefox already has an internal list where it applies fixes for websites (e.g.: setting a custom User-Agent), and the bug report actually comes from it. You can see it by going to `about:compat` page in Firefox.