Having a full-text (both question and answer) search box at the top of your favorites page is quite useful... the search should be local to your own favorites. Perhaps this would be easier to implement than a local-tags feature, which (I predict) would require modification to the db schema. If devs don't want to modify the favorites page, perhaps we could denote a full-text search of our own favorites by typing favorites: in the SO search bar.
I don't know why infavorites: was deleted from the new search capabilities introduced in January, but this functionality leaves a big gap when you have hundreds or thousands of favorites and you only want to find answers relevant to foo
FWIW, I am hacking around the problem right now with:
Stack Exchange Data Explorer Favorites query by tag nameStack Exchange Data Explorer Favorites query by tag name
Stack Exchange Data Explorer Favorites query full-text searchStack Exchange Data Explorer Favorites query full-text search
EDIT NOTE
infavorites:mine works again, but it only searches the title of the favorite.