On some web pages, when I use the scroll wheel on my mouse to scroll down, the page text shrinks, as if I'd done the appropriate gesture on a mobile browser. Reversing the scroll wheel direction enlarges the text. Moreover, the page down key seems to switch me to another tab, rather than moving down in the same page. I first thought that the scroll wheel problem does not affect all pages. However, it now seems that it has stopped misbehaving. I just went back to a firefox window containing a page I thought had misbehaved earlier, and the scroll wheel now behaves normally and so does the page down key. I upgraded to firefox 4.0, and told it to restore the tabs I had had open in the previous firefox version. The scroll wheel had behaved normally in that version. I have not closed and restarted firefox since the upgrade. I don't recall what version I had been running, but I generally upgrade promptly when a new version is available, so I was probably on the latest of the previous series.