sorry, it isn't line nr 306 but 302 (in clean file directly from cvs). But it doesn't matter, i think.
I found a solution even if I don't know where should I look. I made a last try and change the intializing variable originaly from an empty string to "false" string.
Anyway - the checkboxes are now properly shown both in Mozilla and Konqueror.
I put below my cvs diff if someone think its useful:
RCS file: /cvsroot/jboss/nukes/nukes/src/main/org/jboss/nukes/core/modules/block/BlockModule.java,v
retrieving revision 1.19
diff -r1.19 BlockModule.java
< String checked = "";
> String checked = "false";