Fix sourcemaps in JSXTransformer for Firefox

Fixes #1725
> Things seem to work fine if we just use filename.js instead of path/to/filename.js

This sounds to me like "I want to use just filename.js but I can't". I guess it seems obvious enough to me that having the path is better if it works – will anyone see `.pathname.substr(1)` and ask, "why isn't that just the filename?"? At any rate, maybe:

> We could use just the filename, but hopefully using the full path will prevent potential issues where the same filename exists in multiple directories.