How can I determine what is making FireFox slow?

10-11  Source: Network gathering  Views:0 

I am using FF 34.0.5 on Win 8.1 64-bit. It works very well, but occasionally it will get very noticeably slower. When I go to Task Manager, it reports FF is using 29-31% of my i5 3570K 3.4 GHz CPU. It continues to work, and does not use up more resources (ie. not a memory leak). I can easily fix it by closing FF, and ensuring that no FF processes are running in Task Manager. When I next restart, it is back to using a few % points (2-4%) and is much faster.
What tools can I use to determine what it gets stuck on when it hits 30%?
Can you post your troubleshooting data?
Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.
*Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
*Do NOT click the Reset button on the Safe Mode start window
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