It is possible that a handler mapping is missing. By default, the static file handler processes all content.
   The feature you are trying to use may not be installed.
   The appropriate MIME map is not enabled for the Web site or application. (Warning: Do not create a MIME map for content that users should not download, such as .ASPX pages or .config files.)
   If ASP.NET is not installed.

   In system.webServer/handlers:
       Ensure that the expected handler for the current page is mapped.
       Pay extra attention to preconditions (for example, runtimeVersion, pipelineMode, bitness) and compare them to the settings for your application pool.
       Pay extra attention to typographical errors in the expected handler line.
   Please verify that the feature you are trying to use is installed.
   Verify that the MIME map is enabled or add the MIME map for the Web site using the command-line tool appcmd.exe.
       To set a MIME type, use the following syntax: %SystemRoot%\windows\system32\inetsrv\appcmd set config /section:staticContent /+[fileExtension='string',mimeType='string']
       The variable fileExtension string is the file name extension and the variable mimeType string is the file type description.
       For example, to add a MIME map for a file which has the extension ".xyz": appcmd set config /section:staticContent /+[fileExtension='.xyz',mimeType='text/plain']
   Warning: Ensure that this MIME mapping is needed for your Web server before adding it to the list. Configuration files such as .CONFIG or dynamic scripting pages such as .ASP or .ASPX, should not be downloaded directly and should always be processed through a handler. Other files such as database files or those used to store configuration, like .XML or .MDF, are sometimes used to store configuration information. Determine if clients can download these file types before enabling them.
   Install ASP.NET.
   Create a tracing rule to track failed requests for this HTTP status code. For more information about creating a tracing rule for failed requests, click here.

this is the complete error I get. I just installed firefox so that I can use this feature and now it is useless. Error Summary HTTP Error 404.3 - Not Found The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map. Detailed Error Information Module StaticFileModule Notification ExecuteRequestHandler Handler StaticFile Error Code 0x80070032 Requested URL http://www.refworks.com:80/refgrabit/install.xpi Physical Path E:\RefGrabIt\install.xpi Logon Method Anonymous Logon User Anonymous Failed Request Tracing Log Directory C:\inetpub\logs\FailedReqLogFiles Most likely causes: It is possible that a handler mapping is missing. By default, the static file handler processes all content. The feature you are trying to use may not be installed. The appropriate MIME map is not enabled for the Web site or application. (Warning: Do not create a MIME map for content that users should not download, such as .ASPX pages or .config files.) If ASP.NET is not installed. Things you can try: In system.webServer/handlers: Ensure that the expected handler for the current page is mapped. Pay extra attention to preconditions (for example, runtimeVersion, pipelineMode, bitness) and compare them to the settings for your application pool. Pay extra attention to typographical errors in the expected handler line. Please verify that the feature you are trying to use is installed. Verify that the MIME map is enabled or add the MIME map for the Web site using the command-line tool appcmd.exe. To set a MIME type, use the following syntax: %SystemRoot%\windows\system32\inetsrv\appcmd set config /section:staticContent /+[fileExtension='string',mimeType='string'] The variable fileExtension string is the file name extension and the variable mimeType string is the file type description. For example, to add a MIME map for a file which has the extension ".xyz": appcmd set config /section:staticContent /+[fileExtension='.xyz',mimeType='text/plain'] Warning: Ensure that this MIME mapping is needed for your Web server before adding it to the list. Configuration files such as .CONFIG or dynamic scripting pages such as .ASP or .ASPX, should not be downloaded directly and should always be processed through a handler. Other files such as database files or those used to store configuration, like .XML or .MDF, are sometimes used to store configuration information. Determine if clients can download these file types before enabling them. Install ASP.NET. Create a tracing rule to track failed requests for this HTTP status code. For more information about creating a tracing rule for failed requests, click here. Links and More Information This error occurs when the file extension of the requested URL is for a MIME type that is not configured on the server. You can add a MIME type for the file extension for files that are not dynamic scripting pages, database, or configuration files. Process those file types using a handler. You should not allows direct downloads of dynamic scripting pages, database or configuration files. View more information »