Firefox extension does not work with local PDF files

### Steps to reproduce
1. Download latest and install latest build of Firefox extension (eg. from the link at the bottom of the [build output here](https://travis-ci.org/hypothesis/browser-extension/builds/161012437)). _Note: You'll need to download the .xpi file first and then load it via File -> Open File_
2. Open a local PDF file in Firefox
3. Activate Hypothesis
### Expected behaviour

Hypothesis should load the PDF in a new tab using its bundled version of PDF.js
### Actual behaviour

Hypothesis fails to activate and displays an error icon on the badge. Additionally clicking the badge does not show the error details page.
### Browser/system information

Firefox Nightly v52a1
### Additional details

Same steps but with a PDF hosted on a HTTP URL works