hello, single process applications can only use the computational power of one core of your cpu. https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox

Hello

also, if you have 100% cpu in firefox then, probably, you could not do anything else, like to open an application in the same time, like "word" or to play a song in player etc.  :-):-):-)

thank you

If you happen to be using a dual-core CPU and you see Firefox.exe "stuck" at 50%, that means it is using 100% of one of your cores' CPU. I have a dual-core setup and this happens frequently just prior to Firefox crashing.