Can Firefox tell what it uses CPU for?

Each time I open a new page in Firefox, 1 CPU core is maxed out for 30 sec.

Can Firefox give some kind of information about what it uses the CPU for?

about:memory shows Firefox's memory usage details. There's also a button on that page that allows you to minimize memory usage. Source

StatusbarEx is a Firefox extension that will show system information on statusbar of firefox, such as the memory usage of system & Firefox itself, network speed, system power status, etc.

However, it doesn't work 'tab specific'. since Firefox (unlike other web browsers) is using threads instead of processes to open tabs, I doubt this is possible at all. Source

Note: Firefox 16 will have proper built-in profiler.

Disable all your extensions and load a page. If the cpu spike is gone, it's a problem with an extension. You can try enabling them one by one to find the culprit, or try a binary search to save work:

Enable half of your extensions and load a page. If there's still no cpu spike, enable half of your remaining extensions and try again. If the cpu spike appears, disable half of the extensions you've just enabled and try again.

Rinse and repeat.