Font Finder is designed for any web developer / designer that needs to find out the CSS information of any text on the page.
Simply highlight any text element, right-click, and choose font finder. An alert box will appear with all of the styling information you could require including font-size, font-family, color (in both hex and RGB), line-height, letter-spacing, and much more! In addition, you are not told only the CSS font stack, but exactly which font from the stack is currently being rendered.
I'll be updating this extension in the future to incorporate a lot more typographical information, particularly around the new CSS 3 properties.
In this repository I've included the source code for all three versions of Font Finder; Firefox, Safari, and Chrome. I would recommend reading the relevant extension / plugin programming guides for the browser you are interested in before attempting to use this code. Installation should be a simple case of copying the correct folder and using that as your base for your own plugins.
This work is licensed under a Creative Commons Attribution 3.0 Unported License.