I've been dealing with this lag in firefox for the past 6 or 7 years. It's hilarious to me that they still haven't fixed it. Just one of the reasons I've been extremely negative on firefox.
This is the first time I've seen something like this. I use Firefox full time for development and personal use and do a lot of front-end work. ¯\_(ツ)_/¯
Edit: And they have commented out the box-shadow! Hah.