This article describes some causes of a long start up time and steps you can take to improve it.
Problems with a website can cause Firefox to take a long time to start. Try changing your Firefox home page to the default home page or a blank page. See How to set the home page for instructions.
If you set Firefox to use the Session Restore feature to show your windows and tabs from last time, Firefox can take a long time to start if you had lots of websites open the last time you used Firefox. You can choose to open the last selected tab only:
Firefox reads a number of files normally during startup. Some antivirus software may do intensive scans of each of these files during startup and block access until those scans complete. If the problem began after installing, updating, or reconfiguring your antivirus software, contact the software provider for assistance.
Firefox loads extensions when it starts up and many extensions add startup tasks. See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems for steps on how to identify an extension that may be causing the problem.
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If the previous suggestions don't significantly reduce Firefox startup time, see Troubleshoot and diagnose Firefox problems for other steps you can take to resolve the problem.