Firefox no longer works with some versions of Windows XP
This article only applies to Firefox on Windows.
Firefox support is ending for Windows XP and Vista. See this article for details.
Windows XP with Service Pack 2 is the minimum Windows version that meets the Firefox System Requirements. We'll explain how to find out your Windows XP version and what to do if you're using Service Pack 1 or below.
Note: In all cases, it's best to upgrade to the latest version of Windows, if you are able to. Microsoft no longer provides security patches for Windows XP.
How to check which Windows XP Service Pack is installed on your computer
Click on the Windows Start button and open the Control Panel.
Click on the Performance and Maintenance link (Category View only), then click on System to open the System Properties window.
In the System Properties window, select the General tab. Within the information in the General tab, it will tell you which service pack is installed (if one is installed).
Service Pack 2 or 3
If it says Service Pack 2 or Service Pack 3, your Windows system meets the requirements for the latest version of Firefox.
Service Pack 1 or nothing at all
If it says Service Pack 1 or nothing at all, your system doesn't meet the Firefox system requirements. The only popular web browser that is still being updated with security patches for older versions of Windows XP is Opera.
Note: Opera 36 is the last supported version for Windows XP. According to the Opera May 4, 2016 release notes, As of Opera 37, the minimal system requirement for Windows users is Windows 7. Windows XP and Vista will no longer receive feature updates, only security and stability updates.