If you are getting this error and you are running an older version of FireFox then the solution is simply to upgrade to FireFox 3. If you are already on the latest and greatest Firefox than you will need to follow these instructions:

DWA has issued fixes to various maintenance release (MR) streams to move the dwagss.jar file into its Forms file so that it has greater control over the MIME content type returned and can now return the proper MIME Type. This issue was reported as SPR# KMOA79C8WX. Please call IBM support to obtain a fix for this issue for Domino 7.0.3 or Domino 7.0.2 with the latest fix pack.
This issue is fixed in Domino 8.0.1 for customers who upgrade from the DWA7.ntf template to the mail8.ntf template (FYI – There is no separate DWA8.ntf template in Domino 8.x; the DWA mail design is now included in the mail8.ntf). Starting in Domino 8.0.1, the dwagss.jar file is included in the Forms8.nsf database. However, the issue will still occur in DWA 8.x if the DWA7.ntf template is still being used instead of mail8.ntf. A fix for Domino 8.0.1 with the DWA7.ntf and Forms7.nsf is currently under investigation.

Best workaround if Web server is using Web Site Rules:
Create a Web Site rule to override the response headers for the /dwagss.jar file and set the Content-Type response header to application/java-archive

Best workaround if Web server is not using Web Site Rules:
Add the following entry to the <domino data>/httpd.cnf file. This will resolve encountered regressions in DWA after upgrading to this browser:

AddType .jar application/java-archive # Java Archive

Caveat: For Domino 8.0.1 the modification to the httpd.cnf file must be implemented and the dwagss.jar file must be copied to the domino\data\domino\html directory either from another server or the backup directory.

The Domino Web Server should be restarted after this change.

Note: This workaround might cause some security issues if other malicious .jar files reside within the Domino HTML directory or within other documents in a database on the server. See this thread for more details on this topic: https://bugzilla.mozilla.org/show_bug.cgi?id=405643

DWA relies on signed scripts in Firefox for the following functionality:

* Drag and drop in calendar views
* File attachment
* Cache cleaning on logout
* Dialog boxes
* Reuse child window
* Alarm