Prev/Next walk across the page??

[gary]

Using a trackball, I am inclined to position the cursor over the "Next" (message) button and work my way through a thread.

Low and behold I discover that Prev/Next walk across the page with each mouse click. In fact, alternating Prev/Next on a two-message thread keeps them moving further to the right.

Is there a specific reason for this behavior?

(Firefox 1.0 under SuSE 9.1 on a 1280x1024 display.)