This is a security feature of Firefox (and Chrome, Safari, Opera...)
to allow an HTML page to open a UNC path is a security risk. it is possible in IE because it is developed by the same company as Windows (and is probably used by other applications that leverate off of the hooks between IE and Windows)
You can modify the security settings in the Firefox about:config page.
This setting will only allow access to specific files. you can not link to a directory.
if you're interested in allowing access to files in firefox:
1. type "about:config" in the address bar
2. click "I'll be careful, I promise!" button
3. right click on the window and select "NEW" >> "STRING"
4. enter the following preference name / value pairs
>NAME: capability.policy.policyna
mes
>VALUE: localfilelinks
5. REPEAT STEPS 3 and 4 for the foillowing pairs
>NAME: capability.policy.localfil
elinks.sit
es
>VALUE:
http://domain.com (note: for the domain, you can list multiple domains - just separate each with a space)
>NAME: capability.policy.localfil
elinks.che
ckloaduri.
enabled
>VALUE: allAccess
6. Restart Firefox
Note: you can paste a UNC path into these browsers and some will display the contents but it does not work like IE where it opens an explorer window showing the contents of the folder.