What an excellent tool.
Easy to use and works like a charm.
This was my workflow.
I went into firebug.
Selected a piece of text. Copied all the classes from the firebug menu. Pasted them into the extension. And lo and behold. (Dont forget to add comma's between the different classes you want to influence.)
All my text updated without a problem.
It gave the site i am working on a completely different look.