Nested tab numbering per ryanyrzh

This is a super lightweight Firefox extension for Tree Style Tab that overlays tab numbers on the corners of each tab, with two possible counting rules:

• Nested count. Hierarchical numbering that follows the tree structure
• Flat count. Simple sequential numbering across the window

Includes some simple customization options for text location, font style, etc.

Works well with dynamically-colored themes like that offered by Adaptive Tab Bar Color!