Bunch of tweaks and enhancements are on the arch wiki
Two specific ones that can help with performance are enabling OMTC and WebRender
Open up Firefox and about:config
Search for “layers.acceleration.force-enabled”
Search for “gfx.webrender.all” and set to true
http://www.yellow-bricks.com/esxtop/#esxtop-thresholds has some guidance on thresholds for different metrics.
Run esxtop, hit the “c” key to view CPU info and compare the usage with the thresholds.
Here are few from the above link.
Note: if your in esxtop you can hit the “f” key and it’ll give you a menu to specify more fields to display.