overnight train Munchen to Firenze

From Dec. 11, the direct overnight train is expected to be cancelled unless another operator (like Austrian rail) decides to take it over from German rail. You'll have to check back later on the DB site and may not know before November.

When tickets do go on sale, normally about 3 months ahead of the travel date, that's when the cheapest, nonrefundable fares are offered, on a limited number of places. When those sell out, prices rise as high as the full or flexible fare that you also see in the shopping cart.

For all rail travel, the best time to book is as soon as the tickets become available. The prices only ever go up, never down (or to be precise the cheap tickets sell out, leaving the higher price ones only). In most cases tickets are available from 90 days before, so for 26th December they are available from the end of September, BUT THIS IS ONLY A GUIDELINE. The annual timetable change takes place first Sunday in December. Times past that are not yet decided. DB usually fixes their times early, but Italian trains are often not available until a few weeks before the changeover.

Yes, the place to book is DB (German railways): www.bahn.de Rail Europe is a US-based reseller which many people on this site will tell you has limited range and prices with mark-ups.
And don't worry, most people in Europe don't make travel plans or buy tickets until a few weeks or even days or hours beforehand.

There is a Euro Night train EN 235 or 236 from Wien to Firenze dep 19:23 arr 06:07 which can be booked on the OBB

You would just have to take an earlier train from Munchen to Wien

Or you could take the EC to Villach and meet the EuroNight train passing through at midnight.

Train schedule change is the 2nd Sunday of December. Since this one isn't really up to Germany but depends on another operator stepping in, the deal may not be wrapped up on the usual timetable. You wouldn't go all the way to Vienna to catch the currently-operating Austrian night train; number 237 goes through Salzburg around midnight.

From Munich to Florence, you might consider taking a day train. The scenery is quite beautiful and not to be missed.