Firefox 3 will refuse to install an extension if it does not have a secure update URL.
If you wish to bypass this safeguard, do the following:
- In the address bar type "about:config" and press enter.
- Click through the Firefox warranty disclaimer. (amusing)
- Right-click in the list of preferences.
- Select New and then Boolean.
- Enter "extensions.checkUpdateSecurity" as the preference name.
- Choose "false" as the value.