.. and why does it stop running every now and then? I can't see a
pattern to its failures.
There is a page here, on configuring plugin_container.
If you want to configure Firefox, to not isolate the plugins,
they can be run within the main browser process.
http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins
The most likely explanation is, the plugin_container thing isn't
working right. Perhaps if the plugins are run the "old way",
they'll still be running. It could be that just the plugin_container
makes them cranky.
If you make changes to about:config, don't forget to write them
down. You might want to revert them, in some future release
of Firefox.
Firefox checks to see if the plugin is responsive, with a 45 second
timeout. If something caused plugin_container to freeze for 45 seconds,
then Firefox would terminate it, and the plugin itself would appear
to have failed. One question would be, whether some heuristic virus
detection of any antimalware programs, "likes" the new feature.
Maybe something like Zonealarm is interfering ?
Paul