Firebug doesn't support libraries like Flot at the moment and as canvases are pixel-based, there are no objects to debug.
You can
debug the code creating the canvas via the
Script panel, though.
If you want to have support of libraries like Flot, you may
create an enhancement request for it. In that case please provide detailed info on how you imagine the UI for that to look like and provide simple
test cases.
Sebastian