The first problem may cause the other two. Is it 'Firefox cannot be downloaded' or 'Firefox cannot download'? If it is the latter then there are many reasons why a file may download to fail. What works for me is resuming the file download until it completes.

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

You can find the full version of the current Firefox release (90.0.2) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release
• https://www.mozilla.org/en-US/firefox/90.0.2/releasenotes/