In a classic add-on it is slightly more complicated - chrome:// URLs normally cannot be used by web pages, for security reasons. You can however specify the contentaccessible flag for your namespace in chrome.manifest, e.g.:
content myextension chrome/content/ contentaccessible=yes
Then web pages will be able to use chrome://myextension/content/file.swf. The other option would be to register a resource:// namespace, these are always accessible from web pages.

Note that there are no security restrictions preventing web content from including content at resource: URIs, so take care what you make visible there.