One thing I'd like to see is autopromoting folders into the tag search. I have some folders for the most common types of links I save, so entering the folder name into the awesome bar should treat it like a tag.

One feature I can see myself using is grouping related tags and autofilling them. Like if I have the tags python, programming, and language grouped together it should autofill the programming and language tags when I enter python as a tag.

But yeah, FF tags work great.