
The first thing you should try is to make sure that you have the latest Java. At Internet plug-ins, make sure that Allow Plug-ins is checked.

In the Add-ons Manager window, select Plugins. From the Firefox menu, select Tools then click the Add-ons option.

If a web page with Java Uploader or Downloader displays correctly, but does not upload or download chosen files, this may mean two things: How do I enable Java plugin in Firefox Open the Firefox browser or restart it, if it is already running. If you click the Allow now button, you will encounter this problem again when visiting other web pages that use Java. When you click Allow and Remember Firefox adds the current webpage to an internal whitelist, whereby the Java applet will run automatically in the future without further intervention.
