It’s not a GPU driver issue. I have the exact same problem, and have had it on two separate AMD GPUs (one 6970 and the other a 7950) across multiple driver sets. (And yes, I always keep my GPU drivers up to date.) If I disable the Avast FF plugin, my “delete bookmark” text comes back. Otherwise, it’s just a blank button.

This is on Avast 8…I’m steering clear of Avast 2014 until some of its thornier issues are sorted out.