"All differences between O/S's are mostly in the G.U.I (graphic user interface.) "
With all due respect, anyone person saying that has absolutely no clue at all what they are saying.
The difference in the OS has little to do with GUI. Linux is Linux, but you can have several very different GUIs - gnome, xfce, Kde, fluxbox, windowmaker, etc. They are *very* different, but the OS running them is identical. Very different GUIs, exact same OS.
For the non-technical user, the reason to use Firefox with Linux is:
It is free.
It is very stable.
It is very secure, and has almost zero virus/malware risk.
It is very fast.
There are many technical reasons to use Linux instead of Windows, but those four basics are a good start.