Make the chrome extension compatible with Webextension (Firefox)

[gagarine]

Webextension are the coming new extension for Firefox. Their are very similar to the Chrome extension and should be compatible.

Changing the manifest.json by simply adding the following key should make works.

"applications": {
  "gecko": {
    "id": "zotero-connectors@zotero.org"
  }
},

This should help: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Porting_from_Google_Chrome