Very helpful article from Atlassian…
Wiki markup allows you to links to files on the network / server with the format:[file:///c:/temp/foo.txt]
This works fine under Internet Explorer, but Firefox and Mozilla block links to local files for security purposes. If you are happy with the risk of linking to local content, you can override the security policy and also enable linking in Firefox
The instructions for this can be found at http://kb.mozillazine.org/Links_to_local_pages_don’t_work and you may also want to check out the other network preferences.
Please note that you need to use full URL syntax for your link (from http://kb.mozillazine.org)
You also need to use proper URI syntax for local file references. It is not proper to enter an operating-system-specific path, such as c:\subdir\file.ext without converting it to a URI, which in this case would be file:///c:/subdir/file.ext. In general, a file path is converted to a URI by adding the scheme identifier file:, then three forward slashes (representing an empty authority or host segment), then the path with all backslashes converted to forward slashes.