Firefox has a very old bug (see also https://bugzilla.mozilla.org/show_bug.cgi?id=33654) concerning the handling of TEXTAREA elements:

The specified number of rows (ROWS) will not be handled properly. For example, when you specifiy an input field with 3 rows and 40 columns using

<textarea rows="3" cols="40"></textarea>

Firefox displays 4 rows. In fact, always one more row will be displayed. For example the element from above:

Line 1 Line 2 Line 3 Line 4

This is also true for newer versions.

One possible workaround: Set the height using CSS (height) to a fixed value. This is more reasonable than the ROWS attribute anyway, since this way layout and structure will be cleanly separated.