Xscroll

Xscroll is a tiny extension which enables horizontal scrolling by single mousewheel.
X means x-axis and cross in this name.

Scroll horizontally almost elements that have either horizontal scrollbar or truncated text-line in the Mozilla's browser, messanger, chatzila and composer by mousewheel with your preferable modifier key.
Since Mozilla1.7* scrolling text-line does not work.
Let horizontal scrollbar at the bottom of the window scroll horizontally by mousewheel actions without modifier key.
Horizontall scrollbar in GTK+ and some other applications act like this, and I think it is reasonable behavior.
Switch tabs by wheel scrolling on tabs (disabled as default)

If your mouse already has a feature to scroll horizontally, You should not install this.
In the Firefox (or Firebird), this works in the browser's window only.
If your Mozilla was built with an option `--enable-default-toolkit=gtk2', you have to apply a patch, and then rebuild it since gtk2 version can not return correct event target for DOMMouseScroll.

Xscroll does not provide an interface for configurations.
You can customize following values from about:config

extensions.xscroll.modkey (Integer): Modifier key which needed to be held down to activate horizontal scrolling.
All of actions are triggered by the event DOMMouseScroll.
So you have to set mousewheel.withthiskey.action 0 to avoid confliction except using mouse button as modifier.
0 : Alt
1 : Ctrl
2 : Shift (Default)
3 : Meta
4 : Right Mouse Button
5 : Left Mouse Button
extensions.xscroll.delta (Integer): Amount of the scrolling per actions.
The default value is 36.
The value must be between 0 and 360.
extensions.xscroll.direction (Integer): 0 : wheelUp => scrollLeft, wheelDown => scrollRight
1 : wheelUp => scrollRight, wheelDown => scrollLeft
extensions.xscroll.switchtab (Boolean): true : enable
false : disable (Default)

support smooth scrolling.

mozdev.org