The value of the preference 'extensions.update.url' is
"chrome://mozapps/locale/extensions/extensions.properties", which done on
the advice of this page:

  https://developer.mozilla.org/en/XULRunner_tips#Extension_Manager

However, other sources say this value is wrong:

  https://bugzilla.mozilla.org/show_bug.cgi?id=674403#c16

And the following error has been reported by at least one user when
attempting to install an extension:

  Console error: [JavaScript Error: "syntax error" {file:
"jar:file:///opt/xulrunner/omni.ja!/chrome/en-US/locale/en-US/mozapps/extensions/extensions.properties"
line: 1 column: 1 source: "#LOCALIZATION NOTE (aboutWindowTitle) %S is the addon name"}]
    Category: malformed-xml
  Console error: [JavaScript Warning: "WARN addons.updates: Update manifest was not valid XML" {file:
"resource:///modules/AddonUpdateChecker.jsm" line: 476}]
    Category: component javascript