La próxima vez que se te cuelgue cuando visualizes algo en Youtube, Cierra el proceso "Plugin-container.exe", Si al cerrarlo te vuelve a funciona Firefox (Que de echo pongo la mano en el fuego de que ese el plugin-container es lo que te está dando problemas), Entonces ya sabemos un poco mejor donde está la solución (Abajo).
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If you have a problem with the plugin-container, you may be having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?
Whenever you have a problem with Firefox, whatever it is, you should make sure it's not coming from one of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode and select Disable all add-ons. If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and theme. You can troubleshoot plugins the same way.
Whenever you have a problem with Firefox, whatever it is, you should make sure it's not coming from one of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode and select Disable all add-ons. If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and theme. You can troubleshoot plugins the same way.
Si con eso no se te arregla, mala suerte, Se supone que gracias al plugin-container, si se buggea un plugin (Adobe flash o silverlight x ejemplo) el programa entero no se debería buggear, pero vamos, eso es una estafa de container, si se buggea el plugin flash al abrir 20 videos a la vez en youtube, se buggeará firefox por completo, Creo que eso todos los que usamos firefox (Y vemos muchos videos) lo hemos experimentado alguna vez o constantemente hasta una nueva actualización.
Saludos.