Probably not Firefox. Chrome comes with a bundled flash version and does not use the one from your system. So probably you have a bad plugin installed that is being used by Firefox.
Install and run
FLASH-AID to make sure you have the proper flash plugin installed.
Since Firefox 3.6.4, the flash runs isolated inside a process called
plugin-container. This is done to avoid crashing the browser if the plugin crashes.
To reduce CPU usage, install
Flashblock and only allow pages were you need to view flash content.
Ablock Plus might help as well.
Use
BarTab extension. It prevents pages from being loaded until they are needed. So when you start your browser with several pages set as home-page, they don't suck your CPU and memory.
To improve flash performance see
Flash Optimization.
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