The installers now need to be digitally signed in a different way to satisfy Microsoft security requirements for newer versions of Windows. Apparently these are not compatible with XP and Vista, and rather than have multiple signed installers for different versions of Windows, you need to use the internal updater inside Firefox. To trigger that, you can use the About Firefox dialog as described in this article: Update Firefox to the latest release.

Does that work?

See also:

• https://support.mozilla.org/kb/get-latest-version-firefox-windows-xp-vista

jscher2000 said

Apparently these are not compatible with XP and Vista,

They are if you have Windows XP SP3 and updated Vista enough to have patches like KB2763674 installed.

The issue is the browser user agents (on WinXP) does not show if you you have SP2 or SP3 installed. Mozilla is serving a version (43.0.1) that WinXP users can install and then update to current with instead of 43.0.2 to current that many WinXP users still using SP2 cannot install.

I have XP SP3 intalled and am running 43.0.1 firefox. I'm told by help about that I should update to 45.0.1. When I do so, download either doesn't complete, or I am sometimes offered the download for my present version. Are there firefox developers reading these postings? If so, I'm surprised no one is coming up with any remedies.

If you do have WinXP SP3 then you can try the 45.0.1 setup from https://archive.mozilla.org/pub/firefox/releases/45.0.1/win32/ and pick your language and download the larger setup and not stub.

James. Thank you very, very much. I had tried many propossed solutions over a number of days and nothing worked until you offered the following site: https://archive.mozilla.org/pub/firefox/releases/45.0.1/win32/ . The installation completed like magic and help about reports that I indeed have 45.0.1 installed. Have a great weekend.