Firefox demands more memory with each new release and such is the case with their multiprocessor 'feature' that opens 5-6 firefox.exes. Each firefox.exe eats up your system memory and for me there isn't any notable speed increase. If you want to disable this so you only have one firefox.exe then in the Firefox location field put:
In some cases there may also be a browser.tabs.remote.autostart.2 put them both to "False" and then restart your browser.
Note, if you are a developer and dependent on mobile view then this will disable that feature but there are plugin alternatives.