I found that using the extensions FireGestures and Grab and Drag greatly increased my efficiency of navigating my browser.
The former is a very neat gesture addon, which draws a trace of your mouse gesture and uses only four directions (up, down, left, right) so it is much easier to perform a given gesture compared with systems using eight directions because the 4-way system is more robust, so a slight change in the direction of the gesture will not close your tab instead of opening a new one. You can be sloppy with your gestures thus you can perform them very fast.
The latter extension is inspired by Acrobat Reader and lets you drag the webpages with your mouse or touchpad. Especially useful for longer pages because it doesn't cramp your middle finger as scrolling with the wheel would do. You can use flicker gestures (a short flick of a mouse movement instead of a pgup/pgdn button) and momentum scrolling (start to pull the page, then let it go and watch the page as it keeps scrolling).
These extensions are very useful IMHO because using a browser is mouse-oriented (efficiently navigating a webpage with keyboard is greatly inefficient), so why don't give more power to your mouse? The only downside is that you'll start to use it in other programs as well even, if it clearly doesn't work. And in Flash animations neither the grabbing nor the gestures work, of course.
PS. as these addons might not provide visible improvement to the UI, they really increase your productivity and do not forget that the navigation is part of the UI.
PS2. You can install these add-ons anywhere as they mostly go unnoticed until accidental but pleasant discovery by the owner ;-) (compare e.g. NoScript, which wreaks havoc in Firefox.)