My vote is for (Chromium variants), even though Firefox adimirador never used a (variant) of it! At the moment I use Vivaldi and Firefox, I usually open several pages at once (do this in Firefox is deadly) Firefox starts presenting crashes, sometimes open many tabs at the same time the browser crashes completely ...
Vivaldi in turn is equal Chrome (at least here) does not crash when I open multiple pages at the same time, fast and light. I think Firefox fast opening pages and lightweight too, these latest versions are very fast!
I have nothing against Chrome, I've used for years! I think the safest, not otilizo Google Chrome because Vivaldi serving me very well! (Perhaps when Google by the standard form of design material, not going to the settings (chrome://flags ) I can install it again), I see many saying that Chrome comsome much ram, honestly I do not see it!
Doing tests on my own system does not see much difference in speeds, Google Chrome (Chromium variant) renders the page faster, begins to display images with more quickness than Firefox, more by the end of the load does not make much difference, thousandths. ..