When a tab is on its way to being destroyed, if it has set any beforeunload or unload handlers, we run those. Currently, I believe we have a tolerance of several seconds for long running scripts in those handlers. This might affect the performance of other tabs that are running in that same content process. We show the ProcessHangNotification after some number of seconds for long running JS, but the time to wait to show that thing might be too high. Since at this point in a tabs lifetime, the user has made a clear choice ("I want this to go away now"), we might want to be more aggressive about killing the scripts if they're running too long.
See Also: → bug 1336763
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