Trier for a birthday dinner?
Oh dear - Trier is pretty but is not really a foodies' paradise. Like many German towns, you will find plenty of dubious Italian, Greek etc places where the locals go to be reminded of holidays - all best avoided.
The good side is that Trier is sometimes pretty (rather than strikingly beautiful) and it's in the middle of a wine growing area and my best eating experiences have reflected that.
Go to one of the wine cellar type places and you should not be disappointed. Kesselstett Weinstube and Cum Vino come to mindjust now -also Deutscherrenhof which may be the best if a bit out of town. People go there for the wine and the food is authentically local and honest but I like that. In some cases the food may be self-service (a bit like a Bavarian beer garden.
If it's a big birthday however andyou want something totally different, then Schloss Berg (Restaurant Christian Bau) is one of Europe's top restaurants. It's been extensively written up elsewhere - expect to pay accordingly!