Just going from picture to picture using the right arrow (after dismissing the lightbox by clicking on it) for maybe a minute locks up the browser (child and parent processes) on 55.0a1 (2017-05-08) on macOS. I had to force-kill. For example, this URL: http://www.vogue.com/slideshow/met-gala-2017-red-carpet-live-celebrity-fashion#22, but I've repro-ed with other slideshows on this site.  is a whole-machine profile I've done using Instruments, you have to look at the two topmost processes (plugin container and firefox). : https://paul.cx/public/profile-nightly-vogue.trace.zip (unzip and open in instruments).
Ehsan told me it's not possible to share Instruments profiles. Can you take a screenshot instead (Cmd-Shift-4)?
Saving NI flag until :padenot is back. :)
Hrm I can't repro anymore, I just tried for a while and it works fine.
Status: NEW → RESOLVED
Last Resolved: 11 months ago
Resolution: --- → WORKSFORME
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