I had the same issue with Firefox 3.0 in a Windows environment. Replicon has a fix that needs to be implemented on the Web TimeSheet Server. It's just a single java script file that needs to be replaced. Here is the reply I received after emailing their support team.
"We have a patch file that needs to be placed in the installation directory of Web Timesheet which will fix this problem. I have attached the file with this email, please follow the steps given below
- Find the file timesheetclassicgui.js and rename it as timesheetclassicgui.old
- Now unzip the file that you received as attachment with this email.
- Restart the Web Timesheet service
- Clear the temp files and cookies and close any open Web Timesheet windows
- Now launch web timesheet in firefox "