The width/height() method seems to behave strangely in FF 126.96.36.199. When a call is made to the height() method of an element which is hidden and while the user is typing in a textarea box, the cursor is moved to the end of the textarea.
To view the problem, view this page in FF2 and then start typing in the middle of the textarea box below. After the first keypress, the cursor will jump to the end of the text.
If you click the "Toggle Hidden DIV" to show make the element visible and then type in the box, it behaves the way you'd expect. Also, using the css() method works fine (click the "Use CSS" button to use the css() method for getting the height.)