LInux Firefox behaves a bit different from other platforms .. I've noticed the same thing with fonts and some other stuff ... so try to find a bit of a workaround .. I had to do it myself several times ...
Ext GWT is a Java library for building rich internet applications with the Google Web Toolkit (GWT). It includes:
* High performance, customizable UI widgets
* Full theming support with standard CSS
* Well designed, consistent and fully documented source code
* Full remote procedure support using GWT RPC, JSON, and XML
* Support for Java 1.5 features, including generics, enums, and varargs
* Commercial and Open Source licenses available
GWT-Ext is a powerful widget library that provides rich widgets like Grid with sort, paging and filtering, Tree's with Drag & Drop support, highly customizable ComboBoxes, Tab Panels, Menus & Toolbars, Dialogs, Forms and a lot more right out of the box with a powerful and easy to use API. It uses GWT and Ext 2.0.2 LGPL.