Yes you are correct mminton, it is broken. :(

try from : https://ftp.mozilla.org/pub/firefox/releases/49.0.1/win64/en-US/

thanks for report this problem.

Hi mminton, your Firefox user agent shows Windows XP. Are you running XP itself, or a later version of Windows running in compatibility mode?

Just wondering whether the user agent is causing the issue -- the script does check that, I believe, and the link works for me on 64-bit Windows 7.

Edit: "64-bit builds of Firefox are only supported on Windows 7 and higher" according to: https://www.mozilla.org/firefox/49.0/system-requirements/

I was indeed trying to download the 64-bit version from an old 32-bit XP machine, but I was going to use the installer on a different computer running 64-bit Windows 10. I doubt the problem was the machine I was trying to access the link from, since I was able to get the Mac and Linux versions on it.

The https://www.mozilla.org/en-US/firefox/all/ and random locales of the Win64 builds downloaded fine for me including the en-US and en-GB. The OS in use should not matter as I use Linux.

Since you use WinXP this could be a glitch from when Mozilla used to serve Firefox 43.0.1 to WinXP and Vista users. The 43.0.2 to 48.0 were signed in a way that required WinXP SP3 and a updated enough Vista system for those setups to run.

Firefox 49.0 and later no longer has this requirement. However Firefox 53.0 and later may be dropping support for WinXP and Vista.

I tried with Firefox on Windows Vista and it gives the "404 page not found" on all of the some 20 Win64 locales I tried at random. So it is not just WinXP users.