I know of no automated way to get what you want, but this will work as a manual method.
Use Activity Monitor, in Applications/Utilities.
Any web browser will show as just a shell app, with each open page as a child process, so if you go to View menu & select 'All processes, Hierarchically' then click the CPU tab & sort the Process Name column alphabetically, then when you scroll down to Firefox you will be able to see all its child processes indented & quickly analyse which is using most CPU.
You ought to be able to tie this to a specific tab by its name.
Note that the CPU usage in that section of Activity Monitor is "per core" so 100% is "all of one core". You need to divide by the number of cores [real & virtual] that you have to work out the overall CPU% for any individual process.
If you are uncertain how many cores you have, the simplest way to find out is Window menu > CPU History [or Cmd ⌘ 3 ] then simply count the stripes.
Example showing Safari...
[I don't have Firefox, but it should be very similar].
Note that the last 2 visible processes, SafariCloud... & SafariNotification... are not child processes; as they are back at the non-indented level with Safari itself [ignore the little 'lego' icons, use the left-justification of the process name itself to determine indentation.
Also note you can check which view type you are seeing by the Title bar of the window.
Example Core count...
[Don't be surprised if you only see 4 or 8, this is a 12-core + Hyperthreading machine, so it shows 24]