Huge thanks to the Firefox team, for their help on the issue of our long-standing inpage script race condition.
http://stackoverflow.com/questions/38577656/how-can-i-make-a-firefox-add-on-contentscript-inject-and-run-a-script-before-oth
The problem is that we were injecting a `script` tag and assigning its `src` attribute, which triggers an asynchronous fetch request, and does not guarantee execution order! (That was news to me!)
Instead, I'm now assigning the `script` tag a `textContent` value of the script to inject, and it seems to fix the problem!
There is also a Firefox-only API that could solve this whole problem in an even more elegant way, so we might want to expose a code path for that solution later on:
https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Language_Bindings/Components.utils.exportFunction
Allows you to expose an object from one scope to another. There was even talk of creating a polyfill for it that does virtually what we do, message passing between contexts.
* Add mozilla plugin key to manifest
* Move all chrome references into platform-checking module
Addresses #453
* Add chrome global back to linter blacklist
* Add tests
