By default, Firefox 3 prevents users from installing any extensions that do not offer a secure connection for automatic updates.  This prevents users from being victims of hijacked update URLs.

But for developers or people installing extensions they know they can trust, it is a pain.

Luckily it is easily disabled by:

  1. Enter ‘about:config’ into address bar, hit go
  2. Right click somewhere in the list of keys below.  Select New->Boolean
  3. Enter ‘  extensions.checkUpdateSecurity ‘ as the name
  4. Select false as the value
  5. Reattempt installation of your plugin.

Ran into this problem when installing Extension Developer’s Extension while working on Read It Later.

Found the solution at MozillaZine (via Google)

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