Determine which tab in Firefox is using CPU resources?

Resource usage can be monitored via the about:performance page which goes as Task Manager since Firefox 64. It had a major overhaul and now gives much more detailed insight into the performance of tabs and add-ons.

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

The first likely culprit is Flash. Kill Flash - now.

Then it may be rogue Javascript code. Firefox's Javascript Deobfuscator extention lets you watch the count of Javascript function calls :

It is not a measure of CPU usage, but a close enough proxy : find the function with a runwaway number of calls and you will likely have caught the culprit.