When acomputer programmer writes a computer program, that programmer must decide if, and how, the Clipboard is exported to other applications. When something is copied to the Clipboard, it is the responsibility of the programmer to decide in what format it is placed on the Clipboard, and how the information is made available to other programs.
This is done by using a part of the operating system called the Scrap Manager.
The Explorer programmers chose to include font information in text that is exported to the system scrap. the Netscape progammers did not. When you copy text to the Clipboard in Explorer, the original font is preserved. When you copy text to the Clipboard in Navigator, it isn't.
There's no workaround, short of downloading the Mozilla source code and re-writing the routines that export the Clipboard to the system scrap.