Just to be clear. Under normal circumstances, Firefox has all that access anyway. Just like any other app running under X11.
Running it under a different user id means that if it is compromised, there is an extra step required before it can access files. A step which could lead to the compromise being noticed.
Running it under a different user id means that if it is compromised, there is an extra step required before it can access files. A step which could lead to the compromise being noticed.