How do I debug ExtJS 4 app without building every time?

We have an ExtJS 4.2.1 app and use sencha cmd to build.

But during development I don't want to build every time I change something in the code.

But the index HTML file has bootstrap code in it and the locations of images, other js libraries, etc. is different pre and post build.

Also, even the "testing" concatenated code it too long to set break points in firebug.

So how can I run the code and set break points without building every time.

Typically we just run the code files themselves, but as I said, the location in the source tree, and the index.html file has bootstrap code, so that won't work.