Here's Mozilla's Kraken 1.1 JS Benchmark with Firefox 64 built with Clang vs. Firefox 64 GCC 8.2 vs. Chromium 70 (GCC 8.2) vs. GNOME Web 3.30.2 with WebkitGTK 2.22.4 (GCC 8.2).
Again, everything as packaged by Fedora 29 except for the Mozilla build of Firefox.

https://pastebin.com/kEQHwtEi

Summary: Both builds of Firefox scored essentially the same. Chromium 70 was a bit slower. GNOME Web beat all of them by a convincing margin.