I was on a live chat on espn.com last night and found that when I had it open, the browser began to stutter badly (presumably due to GC/CC). about:memory didn't show anything out of the ordinary. To reproduce, open a live chat (see URL above) in a tab and make sure that it is updating live. Leave it open for awhile and you should see the browser begin to start pausing and stuttering periodically after awhile.
You'll also notice that the throbber spins indefinitely, but that's out of scope for this bug, I would think.
If it isn't either of those, we have a few other tools for jank-analysis that I don't know how to use.
You can also try the built in profiler: https://developer.mozilla.org/en/Performance/Profiling_with_the_Built-in_Profiler
We only have a few sample_label in right now. You'll need a local build and a bit of guessing to pin-point where the time is being spent. You can use an external profiler to find the hotspots to help with the guessing. Once you relevant sample_label in the samples during hangs will be labeled in red.
I think this is a case of us needing to throttle pages spinning in an xmlhttprequest loop
I keep accidentally using a non-existent snappy:p4 priority
I've been running with a CoveritLive chat open for the last 15 minutes or so and don't see any noticeable slowdown.