Is there any way to detect flash-plugin crashes in major browsers (firefox, ie, chrome, safari and opera) via javascript?
It depends how you feel about false positives.
You can have a watchdog that make a ajax call "The flash has not crashed", if the flash seems to still be working. And asume that flash has crashed if is not written. This will create false positives if the user close the page before the check.
You can have a watchdog that make a ajax call "The flash has crashed", if the flash seems to not work. This will miss crashes, like crash that kill the whole browser with it.
Maybe you can have both watchdogs so you get a better idea of whats going on.