Follow these steps: 1 - Go to the link provided (Wikipedia) or any other text area. 2 - Press and hold the mouse button inside the text area to select text. 3 - Holding the button, move the mouse arrow up and down multiple times. 4 - The selection starts flickering / trembling. This happens in the latest nightly (r18765) with native text areas.
Sounds similar to Bug 9934.
*** This bug has been marked as a duplicate of 9934 ***