I couldn't find anything in the profiler either. I did narrow it down a bit though. Most of what I was experiencing was sluggishness in scrolling around the page, not in the animations themselves. I realized that in particular, scolling around the top of the page where all the buttons for the modals are. If I hide all of the buttons, the page works great. So somehow something really bad is happening trying to render a bunch of buttons (I made a test page with a few hundred buttons on it and it hung for a few minutes when I tried to scroll). That sounds pretty crazy, so I'm going to assume this is something funky with my configuration and not with Firefox.

Once I got past that, I'm really impressed with how well these animations perform.

(By the way, I haven't used the Firefox dev tools in a while, and they've come a long way! I'll switch back from chrome soon if they keep improving at this rate)