Dev : Extensions : Extension does not install

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This page is part of the extension development documentation project.

Ask your questions in MozillaZine Forums. Also try browsing example code.

Note: development documentation is in process of being moved to Mozilla Development Center (MDC).

This page describes possible reasons for extension installation problems. Intended audience is extension developers, not end-users. If you have problems with installing extensions in general, you should search somewhere else.

If you are extension developer, you can't get your (recently packaged) extension installed and suspect the problem is with your extension, this is the right page.

Covered applications are Firefox and Thunderbird.

This item will be installed...

This problem is almost always caused by mistakes in your install.rdf file. Take a look at that link to see an example of good install.rdf.

Some known causes include:

  • Malformed RDF/XML in install.rdf.. Make sure your install.rdf is valid XML by renaming it to install.xml and opening in Firefox. If it doesn't show any XML errors, it's probably "valid enough" XML for Mozilla. A good idea is to make sure your install.rdf is well-formed RDF too. There's an RDF validator at (it displays some warnings even for correct install.rdf, but that's ok).
  • Wrong information in install.rdf. Triple-check your install.rdf. The following mistakes are likely to cause the "will be installed" bug:
    • Typos in <em:file> tag. For example your jar file is named differently or placed in wrong directory. You may also have wrong directory structure. Another known problem is forgotten slash after directory name in <em:package> <em:locale> or <em:skin>. That is when you have <em:package>content/myext</em:package> instead of <em:package>content/myext/</em:package>, the Extension manager will not install the extension.


Other possible problems are listed here:

  • Not ZIP archive. You might have packed your extension in a fancy format that the EM cannot understand. You should use simple ZIP format for both XPI and JAR files.
  • Mixed case in extension's package name. Use lower case for chrome package name. (bug 257997, bug 132183). Symptoms: Firefox is completely hosed or doesn't open windows that belong to your extension.
  • <script> tags with wrong URL. (Overlayed) window doesn't open [mozilla tries to load the script from unexistent location].
  • Not a valid package. This error is displayed before your extension finishes installing. If you are uploading your XPI package via FTP, you need to make sure that the transfer type of your XPI package is set to binary and not ASCII.