Those threads don't necessarily exist for performance reasons - there can be many reasons one starts a thread from processing UI events, to IO completion etc. I very much doubt Firefox has an easy time saturating your CPU with work outside of benchmarks.
If Firefox smears its CPU usage over multiple threads, that leaves more single-threaded performance on the table for other apps that may need it. So there could still be an effect on overall system performance.
Well yeah - what CPU bound task do you need to be performant? Beyond many tabs - which is embarrassingly parallel - it's all either GPU, network or memory bound.
Firefox failing to saturate your CPU is a win-state.
