You could write your own server script file that takes a filename parameter and a content parameter.
The server script would find the requested file and replace its contents with the new one.
Writing the Javascript that taps into firebug's info and retrieves the useful data would be the tricky part.
I'd personally rather ask the dev team at firebug to supply a function, it shouldn't be too hard for them.
Finally, Ajax sends the filename/content pair to the php file you created.
