I've recently found out about a majorly annoying phenomenon that can be experienced if you launch a performance-hungry application through Firefox, mostly through downloading a file and opening it directly from Firefox.

Why?

Speaking in terms of process trees, firefox.exe is the root process (or parent process) of the child process, say for example Azureus / Vuze. Vuze is known to be rather performance demanding and when it needs resources, it will take them not only from the system itself, but also noticably from the parent process. I don't know if it's common behavior, but usually that shows in Firefox experiencing the occasional couple-of-seconds lags.

The solution? Close Firefox and restart it. That way, all its child processes are raised to the same level firefox.exe was on and you can now browse without any more lags disturbing your streaming videos or anything else

I realized this when I was running Process Explorer by Sysinternals (a company that creates awesome tools for Windows. In fact so awesome they ended up being bought by Microsoft ). It's a great tool to see not only Process IDs, CPU usage, RAM usage and executable names, you can also see dependencies (parent/child processes), CPU time, threads, etc. As a very nice feature it can also show little diagrams of resource usage (CPU and I/O ops) as task icons in your task bar next to the system clock! So much better than the built-in task manager!

Also check out the other tools by Sysinternals. I use Process Explorer and Process Monitor.