Error messages: what they mean and how to fix

In English

• Fix "The bookmarks and history system will not be functional" error message
• Firefox says it's just updated every time it starts - how to fix
• "This Connection is Untrusted" error message appears - What to do
• Warning Unresponsive script - What it means and how to fix it
• "Firefox is already running but is not responding" error message - How to fix it
• How to fix the Update Failed error message when updating Firefox
• How to fix common problems with Firefox OS Developer Preview
• Troubleshoot the "Secure Connection Failed" error message
• Warning Unresponsive plugin - What it means and how to fix it
• Fix the "Could not initialize the application's security component" error message
• Enable SSL to fix the Firefox cannot connect securely error message
• Websites say that Firefox is outdated or incompatible even though it's the latest version