Many modern mice have an enhanced scroll-wheel:
The wheel can go up and down, and be clicked on, as usual. Additionally to that, you can flip it left and right.

The default behaviour for a browser on flip left/right is going back/forward in browsing. This is intended, and most mice have the "flip left" been triggered with less power, because it is more usual to utilize.

Using this buttons make it very comfortable to "sneak" at hyperlinks and get back, like many people use their browser.

The default mouse button numbers are 6 and 7. In Firefox, they are per-default bound as described.

I demand epiphany to extend it's behaviour in the same, expected way (if you have such a mouse, it *is* expected, most times praised in the hardware's manual or feature list).