User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/603.1.30 (KHTML, like Gecko) Version/10.1 Safari/603.1.30 Steps to reproduce: 0. macOS 10.12.4, Firefox 54.0.1 1. Install some web extensions from store or 3rd party sources, for example: a. https://addons.mozilla.org/en-US/firefox/addon/block-images-webextensions/?src=api) b. https://addons.mozilla.org/en-US/firefox/addon/copy-link-title-webextensions/?src=api c. http://support.kaspersky.com/interactive/kpmffplugin/ffplugin?cid=KPMMAC_2.2&utm_source=interceptor&utm_medium=product&utm_campaign=KPMMAC_2.2 2. Relaunch Firefox 3. Load any page and check injected scripts of installed extensions. Actual results: Firefox haven't injected content scripts, so extensions doesn't work correctly. This happens only at launch of Firefox. You can temporary fix this issue by reopening browser window (without main process relaunch) or disable-and-reenable extensions. But after relaunching Firefox's main process issue appears again. Reproducing is not stable (~80%) Expected results: Firefox have injected scripts.

I've noticed that after a launch, ExtensionManager.extensions (resource://gre/ExtensionContent.jsm) doesn't contain all enabled extensions. Therefore, disabling an extension leads to incorrect processing of 'Extension:Shutdown' message: extension.shutdown() called for undefined object.