The problem has been fixed in Firefox Quantum. Its CPU usage in the game is comparable to that of Chrome. Now Firefox is good for GoB again!
The latest Firefox releases (since at least 50.1.0) have abnormally high CPU usage in Game of Bombs (40..50% on good CPUs) even in menu when no game servers or skin tabs had been opened. Chrome and Edge in the same situations have low CPU usage (below 10% for relatively modern CPUs), as expected.
It doesn't seem to affect FPS in the game much. However, it makes CPU hot, cooler loud and battery discharge quickly.
Profiling shows that CPU is eaten by Firefox engine (Gecko) itself, not our scripts. We can't do anything about it.
Until it's fixed, we recommend playing in other browsers (such as Chrome), especially if you're using a laptop.
If extra energy consumption and cooler sound doesn't bother you and FPS in the game stays around 55..60, there is no need to change a browser.