I cannot reproduce this (Firefox 3.5 on Ubuntu):
* downloaded http://tiddlywiki.com/archive/empty.2.5.0.html
* added some text with umlauts to GettingStarted, saved
* reloaded - everything looked fine
* upgraded to 2.5.3 - everything looks fine
Can you provide more details on your setup:
http://www.tiddlywiki.org/wiki/Troubleshooting
-- F.