In the newest version of mozilla .9.7, the favicons appear in the tabs of browser. However, if you go into edit -> appearance -> fonts and change something (font dpi, font size, etc) and then accept the settings, all of the favicons in the tabs except for the current one, meaning the one that has its content visable, change to the Netscape "N" icon. In addition, the title in the tab changes from the name of the web site to (Untitled). This is very odd, as while I understand Netscape hackers do quite a bit of the programing in mozilla, I always thought that mozilla was ment to be 100% comercial free, and Netscape would add onto it. The fact the the N icon is comming up without going to the netscape site suggests that netscape is a default in some way. Nothing terribly wrong with that I in-and-of-itself suppose, just not what, as far as I know, the way it is ment to be.
16 years ago
I can't reproduce this with today's mozilla trunk build. I do see the icon disappear for the active tab, but I don't get any change in the other tabbed icons. (And, no, Netscape is not a default icon in mozilla builds).
*** Bug 116787 has been marked as a duplicate of this bug. ***
Sites that run on the Netscape Webserver serve the Netscape favicon, so I guess you were not seeing a bug in mozilla but a site that runs on a Netscape server. I cannot reproduce this either, marking INVALID.