I'm with you. I use Firefox as my daily driver, but it really makes my dual-core processor struggle (2015 MBP for home, 2017 for work). It's a great browser in every way except performance. They really need to prioritize that.
Are we really saying that Chrome is just more efficient at wasting CPU cycles to show adverts?
Excessive JS usage is probably the culprit behind the move to Chrome. A number of popular sites are so packed with JS that using Chrome is the only way for normal users to achieve reasonable performance.
If the page actually does anything on top of that the problem grows quickly.