As I mentioned before, browsers vary in performance from time to time. If you have a powerful enough PC and fast internet connection you probably won't even notice. My 2012 entry level laptop really shows the differences in browser performance. And my area still has rock bottom performance DSL, with no true broadband expected, ever. They figure this neighborhood has no commercial businesses that need broadband, most TV viewers are on cable or satellite, and most other folks use their mobile data plans.
For most of the past couple of months Firefox performance took a serious hit on my laptop, with the Task Manager showing excessive CPU and power usage. CPU and power consumption were redlined even with only a single window open. It didn't correspond with extensions or Firefox's built in controls for ads and tracking. It didn't affect my desktop with quad core CPU and more memory, but really slowed my laptop to a crawl. But the most recent updates seem to have fixed that problem.
Browser extensions, add-ons, etc., to cope with tracking don't always fix performance issues. Some just substitute one problem for another. Hiding ads and blocking tracking sometimes burdens performance with badly designed extensions or those that aren't fully compatible with the current browser version. Mozilla is pretty good about keeping an eye on permitted extensions and occasionally they'll disable one that violations terms of service for privacy or performance.