This is an easy benchmark for rejecting a lot of selectors:

http://bholley.net/testcases/style-perf-tests/perf-reftest/b...

Firefox with STYLO_THREADS=1 gets about 160ms on my machine, which is basically parity with safari and chrome. With the parallelism, Firefox gets 40ms. :-)

You can also simulate sequential mode in recent nightlies by ctrl-clicking and loading the tab in the background (we disable parallelism for background loads).