Looking at the addons/extensions in play from the more system details on the upper right of this question I'd suspect the following plugin is doing it's job too well and needs to have NetFlix and TVTropes whitelisted.

Cookie AutoDelete 3.0.1 (CookieAutoDelete@kennydo.com)

Make sure that you do not run Firefox in Private Browsing mode.

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

A detail like a website remembering you (log you in automatically) is stored in a cookie.

You can use these steps to make a website recognize and remember you:

• you can create an "Allow" exception to keep specific cookies, especially in case of secure websites and when cookies expire when Firefox is closed.
• Options/Preferences -> Privacy & Security
  Cookies and Site Data: Exceptions

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Cookies
• do not clear the Site Preferences

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an "Allow" exception you may want to keep