This happens in Firefox 3.0.10 Ubuntu.
It seems SO defines that input box as a fixed size rather than giving it a pixel width.
That works fine for IE on Windows with default DPI and font size settings. It also seems to work on Chrome. But Firefox seems to use a larger default font than IE. You can also change it which will affect it.
This most likely is related to a few other css bugs on SOFU on non-windows platforms.
The most commonly noticed is on the login page.
Its because SOFU uses
This problem appears on both Linux/BSD and OS X.
Try installing the