Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0 precursor to the STR: you need a microsoft account - go to https://sysdev.microsoft.com/en-US/Hardware/signup/ - sign in with your ms credentials - click on the drop-down button next to "please find your company" - firefox hangs for a prolonged period when switching off e10s the dropdown field opens in a near instant. i wasn't able to capture a profile with nightly since it was frozen/not responding indefinitely until windows offered to kill it, but with release the freeze was "just" few seconds and the gecko profiler worked: https://perfht.ml/2qwvqUs

Tested on 55.0a1(BID 20170609030207, macOS 12.12), Firefox hung and no response for several minutes. From stack trace, it seems main thread was busy doing restyling. Attach OSX sampler stack trace for further investigation. TLin, could you help to investigate what's wrong here?

This should be a dup of bug 1118086.