How to disable the Forward/Back buttons on my mouse

Start the program xev in a terminal. Move the mouse inside the xev window; you'll see a lot of stuff scroll by. Press each button in turn. Then switch back to the terminal window and press Ctrl+C. xev shows a description of each input event, in particular ButtonPress and ButtonRelease for mouse clicks (you'll also see a number of MotionNotify for mouse movements and other events).

It's likely that your forward and back buttons are mapped to mouse buttons, maybe buttons 8 and 9:

ButtonPress event, serial 29, synthetic NO, window 0x2e00001,
    root 0x105, subw 0x0, time 2889100159, (166,67), root:(1769,98),
    state 0x0, button 8, same_screen YES

If that's the case, remap these buttons to a different action in your browser, if you can. Alternatively, you can remap the buttons to different button numbers which your browser doesn't react to or disable the buttons altogether at the system level. To do this, put these lines in a file called ~/.Xmodmap:

! Remap button 8 to 10 and disable button 9.
pointer = 1 2 3 4 5 6 7 10 0

Test it with the command xmodmap ~/.Xmodmap. Most desktop environments and window managers run this command automatically when you log in; if yours doesn't, arrange for it to run when X starts.

It's also possible that your mouse sends a keyboard event when you press these buttons:

KeyPress event, serial 32, synthetic NO, window 0x2e00001,
    root 0x105, subw 0x0, time 2889100963, (957,357), root:(2560,388),
    state 0x0, keycode 166 (keysym 0x1008ff26, XF86Back), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

In that case, put lines like these in ~/.Xmodmap:

keycode 166 = NoSymbol
keycode 167 = NoSymbol

You could use xev to find which key maps the button maps to and use the code below (in $HOME/.xsessionrc) to map it to something you are not using.

xmodmap -e 'keycode THE_CODE_HERE = XF86Launch1'

Alternatively, you should be able to do that from a GUI of you use Gnome/KDE or any other modern desktops.

Yet another option would be to hack the xorg.conf to remove the button definitions.

just a quick command to fix it, not really different from other answers:

xmodmap -e "pointer = 1 2 3 4 5 6 7 0 0 0 0 0 0" #the fix

testing

xmodmap -pp # check the changes
xterm -e xev # test the nullified buttons

# to restore in case you are going to play some game that uses them
xmodmap -e "pointer = 1 2 3 4 5 6 7 8 9 10 11 12 13"

you can put the fix cmd at startup apps.

better would be to use xdotool to check if active window is any of your browsers and call xmodmap on demand to disable/enable'm, but that would require some scripting :>

I saw no reason to keep buttons above 7 enabled, any reason?