I can think of several reasons:

• Reverse engineering something is much less fun than creating something new
• ReactOS (as well as mono and wine) are destined to be playing catch-up with whatever Microsoft does. This puts them always one step behind their “competitor”.
• Even further, Microsoft can always make it harder for immitators, by introducing complex new libraries and APIs.
• And even if you manage to get over the previous hardles, Microsoft can always sue you for “copying their intellectual property” , effectively killing all your hard work.

I applaud what the ReactOS guys are doing and wish them the best of luck, but on the other hand I believe they have chosen a very difficult path.

for Firefox specifically, Debian chose version 10.0 because it's an Extended Support Release version (http://www.mozilla.org/en-US/firefox/organizations/faq/) . In order to be able to support it with security fixes in the next 2-3 years. It makes perfect sense if you think about it.