Since Mozilla Firefox 3.6.4 a new feature has been added to make browser plugins, such as Adobe Flash player, run in a separate process so that if they crash, they do not crash the whole browser (which might have multiple tabs open).
The problem is that the plugin wrapper, program called plugin-container.exe seems to have poor performance and can make an older PC (Pentium 4 2GHz) use 50% or more of the CPU just rendering a single flash animation on a web page.
The problem still exists in the latest version of Mozilla Firefox 7.0.1
To disable the plugin container and run the plugins in the same process as the main browser do this:
Enter about:config into the address bar
set dom.ipc.plugins.enabled to false