How to track Firefox's Internet Usage for each tab?

I am not aware of a tool that would do exactly what you describe. However, I would track Firefox network activity as follows :

• Go to Tools > Web Developer > Browser Console. At top right, unselect all elements but XHR and Requests. This will allow you to see all requests transiting by Firefox, may they come from any tab or add-on. Checking a request, you can find what amount of data was transferred in Headers tab, and who is the certificate owner in Security tab.

• Open a new tab and enter "about:performance" in URL field. This will display the Firefox task manager. It does not help you with bandwidth, but display memory and energy impact used by each tab / add-on. This gives already some good ideas of what is heavy and thus probably needs bandwidth to be fetched. It also allows seeing "subframes" for all tabs, which is useful for identifying the request whose certificate owner is unknown to you.

I hope this can help you to identify what is eating your resources, even if it's very manual...

Had the same problem and found a solution:

Find IP with increasing data usage in about:networking#sockets. Then find corresponding active tab by URL in about:networking. (Filter by column "active" and use ping or something to look up the IP address.)

It was an old tab with a live webcam...