I had the same problem. It seems to be an issue with Gallery 1.2 and the .NET framework 2.0. I swapped the web application back to use .NET Framework 1.1, and it is now working fine, also with Firefox.

The problem is that the name generated for the input fields are for example "gallery1$currentpage", but the JavaScript is looking for an element with name "gallery1:currentpage". Interesting that the script works in IE, it obviously assumes that the name "gallery1$currentpage" and "gallery1:currentpage" is the same. With .NET framework 1.1 the name is generated with the colon, not with the dollar sign.

Hope this helps