Map Command Key to Alt. Add the following lines to ~/.xmodmaprc.
! Set the left Apple key act as an alt key.The following command enacts these changes.
keycode 115 = Alt_L Meta_L
add mod1 = Alt_L Meta_L
xmodmap .xmodmaprc(These changes can be set to begin at boot time. Gnome notices the files and will ask you if you want to use it. There is an earlier post here on getting it to work in XFce4.)
Change Accelerator Key in Firefox. Launch Firefox. Type about:config in the location bar and hit enter. Set the following two attributes as follows.
ui.key.accelkey 18The first defines the accelerator key to be the Alt key, while the second disables accessing the menu from the keyboard, which is the usual behaviour for the Alt key.
Now restart Firfox. The menus should reflect the change by reading "Alt+" instead of "Ctrl+" in defining the shortcuts.