What would help is to activate the Gecko profiler[1] when the performance issues occur. The cause of these performance issues might not be in Firefox itself, and the Gecko profiler would allow to find out where is the issue.
With many tabs (>= 100 or so), it becomes really sluggish. For example, opening a link in a new tab can take approximately five seconds. Chrome, which it is based on, does not have this issue. Chrome is however unusable (with many tabs) for other reasons (lack of lazy loaded tabs essentially causing a DoS for many minutes when starting it, at least on Windows).
Maybe the answer is contained in this statement?
Only thee of those are active on more than one website. If Web Extensions are suppose to be this great improvement for security and performance, extensions themselves shouldn't matter, or should at least show up in about:performance if there are issues.
I will attempt to come up with a test case. Mostly I just noticed when resetting Firefox or with a new install, "hey this is snappier than usual" until I install uBO. Not sure if it's resources, UI responsiveness, or rendering speed. uBO is awesome, and I appreciate all the work.
I'm not complaining, and if you're confident in your overhead times feel free to disregard. Just unscientific observations. I still love uBO and use it across the board.
Vivaldi was faster, but has a lot odd bugs, it's not OSS, and has no real public bug tracker.
After some recent UI lockups, I decided to try FF57 again. Most of the extensions I use have been ported, but after just a day or two, I started to hit real performance issues again. I'm not ready to give up on FF yet, but I do feel like many of the performance improvements might have been overrated.