Developing locally in Firefox 3

You can develop locally in FF3 by modifying the security permissions. You can do this by following these steps:

1. Type about:config into the Location Bar (address bar) and press Enter
2. Click the “I’ll be careful, I promise” button
3. Change the security.fileuri.strict_origin_policy:

True
Local documents have access to other local documents in the same directory and in subdirectories, but not directory listings. (Default)

False
Local documents have access to all other local documents, including directory listings.

More information here and here