I'm assuming you mean basic type authentication - if so, yes it is. If you use Wireshark to sniff the traffic from the box you are running your browser from, you'll see a header called Proxy-Authorization followed by a base 64 encoded string. If you take that encoded string and decode it you'll be able to see your username and password in clear text. At least, it worked for me when I did it just now.
If you're trying to sniff someone else's password that's a different scenario and my advice is: don't.
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