Production Firefox extension will not install from local .xpi file

I am receiving the following when trying to install my Firefox extension build.


## Steps to Reproduce:

1. Clone [repo](https://github.com/hypothesis/browser-extension) at commit ea8fa71015616587e9e0cb17fd0f552d05c9094f.

2. On Win 10, using MINGW64 shell provided by `git bash` (with administrative permissions), use Chocolatey to install make tools via the command `choco install make`

3. `git checkout -b my-branch` in order to make a new local branch.

4. Delete all files in `build` folder.

5. Commit changes locally on `my-branch`.

6. Make with command `make SETTINGS_FILE=settings/firefox-prod.json`. Output appears to be successful:

```
$ make SETTINGS_FILE=settings/firefox-prod.json
node_modules/.bin/browserify -t babelify -d src/background/index.js > build/extension.bundle.js.tmp
cat build/extension.bundle.js.tmp | node_modules/.bin/exorcist build/extension.bundle.js.map >build/extension.bundle.js
node_modules/.bin/mustache build/.settings.json src/manifest.json.mustache > build/manifest.json
cp -R node_modules/hypothesis/build/* build/client/build
rm build/client/build/manifest.json
tools/template-context-app.js build/.settings.json | node_modules/.bin/mustache - src/sidebar-app.html.mustache >build/client/app.html
tools/template-context-settings.js build/.settings.json | node_modules/.bin/mustache - src/settings-data.js.mustache >build/settings-data.js
cp src/unload-client.js build/unload-client.js
cp src/pdfjs-init.js build/pdfjs-init.js
cp -R src/vendor/pdfjs build/pdfjs
cp -R src/help/* build/help
cp -R src/images/* build/images
cp -R src/options/* build/options
```
7. Zip all files in build directory into `build.zip`

8. Attempt install of the file from the previous step on `about:addons` page.


I'm happy to include the zip file here, but did not want to accidentally pass on any sensitive data inside it.