IE and Mozilla are listed in the EZSrve guide as examples of supported web browsers. I just installed Firefox 3.0.3 and it brings up EZSrve 1.56 with no problem. EZSrve refreshes most of its screens every 2-3 seconds. Is it possible the performance of your system/network is such that a full EZSrve screen cannot be displayed in that time frame. What is displayed in the lower left corner of the Firefox window.
Try clicking on the Time/Macro Icon (second from top on left side, looks like a clock with a green check mark) and see if that screen will display completely. The Timers and Macros screens are not refreshed on an elapsed time so they may have time to display if network or system speed is a factor.