This is from bugzilla bug:

http://bugzilla.mozilla.org/show_bug.cgi?id=83376

From Bugzilla Helper:
User-Agent: Mozilla/4.0 The lizzard 0.9x rules! (compatible; MSIE 5.0; Windows
98; DigExt)
BuildID: 2001-05-27

I've modified the UA string of the browser. Since then, no way to start
browser again because of an error message which says: "Java Plug-In for
Netscape should not be used with the Microsoft Internet Explorer. Please
use the Java Plug-In for Internet Explorer instead." (see attached
image, text in German - probably because it's taking the Java Plug-In
from my StarOffice 5.2 which is in German; my Windows is also in German
by the way, maybe this is the cause.)

Reproducible: Always
Steps to Reproduce:

1. I added exactly the following line in the prefs.js of my profile:
user_pref("general.useragent.override", "Mozilla/4.0 The lizzard 0.9x
rules! (compatible; MSIE 5.0; Windows 98; DigExt)");

2.Launche Mozilla by clicking on its icon

I have confirmed this happens on win32.