Can you please try running firefox from terminal by typing "firefox --safe-mode" without the " and see if it still uses that much cpu% or if it goes to normal usage. either way can you give us the top readings again with it running in safe-mode.
** Changed in: firefox (Ubuntu)
Status: Unconfirmed => Needs Info
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firefox using over 90% of CPU
https://launchpad.net/bugs/78786
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