If you go to about:memory and click on "Measure" you can get some information about what is in memory. In addition to web pages, Firefox uses child processes fo

If you go to about:memory and click on "Measure" you can get some information about what is in memory. In addition to web pages, Firefox uses child processes for things like the new tab page, extensions, talking to the GPU, and some other things. It also uses a preallocated process like another commenter suggested, to speed up opening a new page.