A common reason for this message is due to running Firefox in Windows XP or Vista compatibility mode on Windows 7 (also on 8 and 10). Due to this you may actually be running 32-bit Firefox 52.9.0esr as that was the last to still run on Windows XP and Vista.

See how to check and disable this at https://support.microsoft.com/en-ca/help/15078/windows-make-older-programs-compatible

You can also right-click the desktop shortcut for Firefox and check properties.

it doesn't help. Ensured again that Compatibility mode is left unchecked. None of the checkbox in Compatibility tab is selected. Also previously tried running the same in Windows 7 compatibility mode but that also does not help unfortunately.

So where are you downloading firefox from then?

https://www.mozilla.org/en-US/firefox/new/

installed a much lower version of firefox release 36.0 and successfully installed. Still the newer versions are unable to install.

Hi vdas, could you check the Compatibility settings on Firefox? Find the icon you use to start Firefox, and either:

• Right-click a desktop shortcut, then click Properties
• Right-click a Firefox icon pinned to the Taskbar then right-click Mozilla Firefox, then click Properties

You should clear all the boxes on the Compatibility tab for best results (giant screenshot below).

Any difference the next time you start Firefox -- can you update?

Note that the last version for Windows Vista was Firefox 52 esr, so if the installer thinks you have that, you should be able to install one of these:

32-bit U.S. English: http://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/en-US/

64-bit U.S. English: http://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win64/en-US/

If you prefer a different language, click the parent directory button (..) and find the correct folder there.

Assuming 52esr installs, please check its compatibility settings as well.