• The updateURL uses https, or there is no updateURL at all (which defaults to addons.mozilla.org which is https)
  
• The updateURL uses http and the updateKey entry is specified which will be used to verify the data in the update manifest.
  

In the update manifest delivered from the updateURL the updateLink must be specified in one of the following ways:

• The updateLink to the XPI file must use https
  
• The updateLink can use http and you must include an updateHash for the XPI file using sha1, sha256, sha384 or sha512 hash algorithms.
  

• javascript.options.showInConsole = true. Logs errors in chrome files to the Error Console.
• nglayout.debug.disable_xul_cache = true. Disables the XUL cache so that changes to windows and dialogs do not require a restart. This assumes you're using directories rather than JARs. Changes to XUL overlays will still require reloading of the document overlaid.
• browser.dom.window.dump.enabled = true. Enables the use of the dump() statement to print to the standard console. See window.dump
  for more info. You can also use nsIConsoleService
  from privileged script.
• javascript.options.strict = true. Enables strict JavaScript warnings in the Error Console. Note that since many people have this setting turned off when developing, you will see lots of warnings for problems with their code in addition to warnings for your own extension. You can filter those with Console².
• extensions.logging.enabled = true. This will send more detailed information about installation and update problems to the Error Console.

• Your server should serve OpenSearch plugins using the MIME type application/opensearchdescription+xml.
• Be sure that your Search Plugin XML is well formed. You can check by loading the file directly into Firefox. Ampersands in the template URL need to be escaped with & and tags need to be closed with a trailing slash or matching end tag.
• The xmlns attribute is important, without it you could get an error message indicating that "Firefox could not download the search plugin from: (URL)".
• Note that you must include a text/html URL — search plugins including only Atom or RSS URL types (which is valid, but Firefox doesn't support) will also generate the "could not download the search plugin" error.
• Remotely fetched favicons must not be larger than 10KB