See https://perf-html.io/public/02e6fd158152f833f33095b1be7937aa1d7f1992/calltree/?thread=2 for a profile capture. Steps to Reproduce: 1. Load a Facebook-hosted Words With Friends session 2. Tools -> Web Developer -> Network 3. Go away for a while, and watch memory usage keep growing 4. Playing on another machine (which syncs game state) also seems to spike this rather quickly Actual Results: CPU can spike to 16%, without actively doing anything, and memory can soar to 1500+MB Having looked at the profile linked to above, :mconley thinks one of the issues at play here is that "netmonitor is causing synchronous reflow in the parent process." The other issue is likely bug 1331680. Not sure if this is also related to/dependent on bug 1350227?

Sorry, should've mentioned: Build identifier: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0 (I can retry in a later nightly, but right now auto-update is broken on nightly due to add-ons, I'm told?)