How to find out which Firefox tab is using most CPU or memory?

Opening the browser to about:memory can show per-tab memory usage.

Further reading

• Techzine article about memory usage.
• Firefox about pages.

For Firefox, I've made an addon which displays tab memory usage in the corresponding tab's title.

It provides user with tab related stats and memory usage...

https://addons.mozilla.org/firefox/addon/tab-data/

Features

• How many tabs are currently open
• How many tabs have been opened during this Firefox session
• How many tabs have been opened since the addon was installed
• Displays tab memory usage in title
• Preference to toggle tab memory usage on/off
• Preference to set interval between memory usage collection

The BarTab extension for Firefox (not available for the latest Firefox versions) might be useful for improving the situation:

BarTab can intercept when tabs are loaded in the background or restored after a browser restart and will only load the content when the tab is actually visited. It also allows you to free memory by unloading already loaded tabs, either manually or automatically.

Otherwise, you might switch temporarily to Google Chrome, where Shift + Esc displays the per-tab usage: