I gave it a look in Firefox v34.0.5 and it looks great (http://d.pr/i/1hJP2). Updated to Firefox v42.0.0 and it also seems to be working OK here (http://d.pr/i/123Sw). I’m not sure what v43.0.3 is as I can’t seem to get Firefox to update to anything newer than 42.0.0. Is 43 a beta version?
Looking like some sort of encoding problem with FF though as @Turtle points out.
Interesting. Downloaded and installed 43.0.3 from their site since the updater didn’t pick that one up. Loaded up the site again and I still get the correct umlauts: http://d.pr/i/Q41m
Could it possibly a font problem? If Firefox was relying on a non-unicode font (or a unicode font which was in some way defective) for the page created in Tesla but not for the pages created in other themes, this could cause it to display some characters wrongly.
This could also be a host issue. I can not remember the exact reasons why, but we have had a few tickets about this and it always came down to something the host had to either switch or turn on. I will try to do a search of our old tickets and find out what.
Edit - wow, ok, a ton of tickets came up, going to be a while, not sure if I will get this done or not.
I had similar problems (not with the Tesla theme) some time before. It was caused by a PHP configuration on the server. In one case it was a Strato server which supports a special PHP acceleration, whatever that was (even the hoster couldn’t give me information what this is). I had to remove the PHP acceleration. In another case it was the PHP version, I had to go back to an earlier version.
Perhaps checking the PHP configuration is a starting point for fixing that issue.
I recognized, that on the page “Impressum” all umlaute are shown correct, also the ü. So I wonder why this could be. Than I remembered, that I copied the text for the page “Grusswort” from a MS Word-document. After I overwrites the words containing an ü in the text stack and transmitted the page again on the server, everythings working fine now.
@HGBM: Glad it was a super easy fix! Copying and pasting can be problematic when doing so from a Word doc or a website. All that extra formatting that the app or site already have on the text carries over. Best thing to do often is to retype the text, or use RapidWeaver’s wonderful Paste As Plain Text feature in the Edit menu. This solves most problems like this. Though who knows with Word, right? It does some weird things to text sometimes. But generally this should strip off that excess formatting, though if Word is just using a different character it may not work.