This is just a personal experience with various web browsers in my Linux Desktop. I run a very old Laptop ( Samsung RV508 ) - [Intel Pentium(R) Dual-Core + 2GB RAM ]. I use the modest Linux Desktop ( LxQt ) on PCLinuxOS 2018. Below, I list out the memory usage + CPU utilization of major web browsers.
Mozilla FireFox Quantum Web Browser - 61.0.2( 64bit )

Google Chrome - 68.0.3440

Opera Browser 55.0

Iridium Web Browser - 2017.10 ( based on Chromium Source Code )

Vivaldi Web Browser (1.15.1147.64)

Falkon Web Browser - 3.0.1

Brave Web browser 0.22.21

Apart from these, Opera seems faster than other browsers when I get into production desktop usage such as doing programming, listening music and browsing many many tabs. Google chrome and Firefox eats my CPU faster than Opera.