URL: https://twitter.com/

Browser / Version: Firefox 57.0
Operating System: Mac OS X 10.12
Tested Another Browser: Yes

Problem type: Site is not usable
Description: In private browsing mode, Direct Message does not work.
Steps to Reproduce:
0. Open https://twitter.com/ in the private browsing window.

1. Click "Messages" pane
2. Direct Message in-content popup is shown, but there will not show the content with error message into the devtool's console: "TypeError: this.database is undefined
   [Learn More]"
3. Mysteriously, this is not reproduce always. Sometimes this works correctly.

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