The latest EXE installers are signed differently and are incompatible with Windows XP Service Pack 2. Since the website cannot detect whether you have Service Pack 2 or Service Pack 3, it assumes the worst.

To update an existing installation of Firefox, please use the internal method, either:

• "3-bar" menu button > "?" button > About Firefox
• (menu bar) Help > About Firefox

This is an evolving issue, so it could change at any time, but for now, that's what I suggest.

To download from a site that doesn't check what OS you're running, you can use this server: http://archive.mozilla.org/pub/firefox/releases/38.5.2esr/

You would have to spoof the user agent temporarily to at least Vista (Windows 6.0) to make the server offer a Firefox 38.5.2 version.

• Mozilla/5.0 (Windows NT 6.0; rv:38.0) Gecko/20100101 Firefox/38.0

Otherwise you can try to update to 38.5.2 via "Help > About".

• UAControl: https://addons.mozilla.org/firefox/addon/uacontrol/

• [/forums/contributors/711730] Windows XP / SHA-1 Issue

Thanks for the super-fast reply jscher2000.

I prefer to download the full file each time so that I can revert to an earlier version if necessary. (I have Win XP Service Pack 3 on this computer.)