Good for you. The 'oh-so-slow' regarding FF was meant as satirical btw. With what I'm using it for, it doesn't lag. Maybe because of https://github.com/graysky2/profile-sync-daemon mostly, and some other fine-tunings, which make it not touching the filesystem over and over again. I really recommend PSD. Not even insane amounts of RAM, it usually takes about 4 to 5GB, rarely going to 8, then shrinking back a while after closing too much tabs. Imagine that!1!! It's all about some sysctl settings :-)

Sideberry (Tree-Style-Tabs like extension) was the ugliest offender there. Though that may have been me misconfiguring it. OTOH I didn't manage to find settings where it didn't do that, and still looked like I wanted it to. At that time, maybe a year ago, I've thought of it as potential 'instant ssd-killer'. Couldn't be bothered. Deinstalled. Now FF has some basic version of vertical tabs by Mozilla itself. It suffices(for now).