I've been using this for many years, but with the first update of Firefox 43, my browser became pretty much unusable. It lagged constantly, had script issues, and would hang for 30 seconds to a minute every time I opened or closed a window, tried typing into a search bar, etc.
I am running the following addons:
Better Gmail 2
Classic Theme Restorer
Search in Google Scholar
In addition, I was running Google Redesigned, but a painstaking one-by-on elimination process made me realize that Google Redesigned was my culprit. I'll definitely try a new version if one is released, but for now this is going to stay disabled.