I wonder if you enable Windows authentication for that site from Firefox if that might do the trick:
1 Open Firefox.
2 In the Location Bar, type about:config, and then press Enter.
3 If a warranty message opens, click I'll be careful, I promise!.
4 Double-click the network.automatic-ntlm-auth.trusted-uris preference.
5 In the Enter string value box, type your Web Host URL using the format http://hostserver:port.
6 Click OK.
Thank yoo Stefanos,
first of all i've to say that my problem has been solved with a new installation on a "more standard" configuration ie.
win2003 srv, IIS 6.0 and the last versione of Firefox.
In this way after the autentication request , i can normally access and work with my doc. , the same is for Safari.
I look for the Firefox extension you suggest to me but is not applicable at the last Firefox version, and i don't know if it works out of windows environment, i will test it when available for ff 3.5