With Firefox 2.0, you can right-click on the frame you want to validate, then choose This Frame->Open Frame in New Tab. You can then switch to that tab and validate the individual frame.
I hope this makes it easier! I do not know of an easier way.
The big issues here is not knowing if there are iframes on a page, and if there are more than one. There are other validators which do check iframes source code as well so hopefully the writer of the plugin can include this at some point.