The Hotmail change, effective 1 Dec 2003, has been publicised for at least two months. Perhaps you missed it.
The Microsoft organisation provides you with a FREE service that you clearly find useful. If they chose to update it from time to time and "retire" legacy programs that is their right. The IT world is fast-moving; stay with it or get left behind.
There are other Webmail providers you could try that would work with Netscape 6. Try a Google search.
BTW, I use OE 6 with XP. It's fast enough for me. OE 6 was brought out because it was (is) smaller and faster than 5.5. It may well be the final version.