Others have reported problems with Java in the web viewer. Perhaps the following information will help: FileMaker has a bug with modal dialogs in Java applets displayed in the web viewer.
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
Thank you very much - I'll check these out - much appreciated