In the past I've found about:memory to be almost useless for pinpointing leaks. It's segmented into processes and types of allocations, and only the deeply nested sub-entries say which tab the memory belongs to. And even then, I don't know whether Firefox, the site, or an extension is causing memory bloat. I end up resorting to just blindly closing tabs or restarting the browser.

I might try about:performance more, but in the past I recall it being similarly almost useless for pinpointing sites eating CPU.