And apparently they don't catch serious problems in their build system on their own (profile feedback pass timing out and truncating data)
Overall not a great result at all for Mozilla. Where is the regression testing for performance? They are not data driven?
GCC aggressively size optimizes cold regions, and LLVM doesn't bother. This would be pretty easy to fix, but outside of binary size, need to prove it actually matters (the test harness here is a pretty darn old CPU)
The "don't catch serious problems in build system" is the bigger worry, imho.
GCC does "almost full LTO" with partitioning, while clang does thinLTO that does make most of code size/speed tradeoffs without considering whole program context, so it may be interesting to get both alternatives closer in code size/performance metrics.
I have got Firefox developer account of level1 and I am looking into official benchmarking architecture which I have now updated to GCC 8 with LTO+PGO.