do you have anti virus software? It used to be that they were good for corrupting downloads in Windows.

Are you sure you are downloading an OSX version of the software? The download is a DMG file?

I don't have anti-virus software running. I'll check to make sure I'm downloading the correct type of file, thanks.

Yes, it's a DMG file. Once it downloads, I get this message when I try to click on it.

This article is not for Thunderbird, but I think the solution offered might fix things for you. https://secure.clcbio.com/helpspot/index.php?pg=kb.page&id=323

Thanks for the article. I followed instructions but still not working. I think it's time for a trip to the Apple store. Cannot wait to dump this stupid Mac and get back to a windows machine!

Solved! I had to open a terminal on my Mac and typed "sudo spctl -- master-disable, enter my password, and now I can download whatever I want. Found it on Youtube.

You obviously did not read to the end of the article where is says under Mac OS Sierra 10.12.x

The premise is the same as shown for Mac OS 10.9.2 through 10.11.x except that the steps to enable the "Anywhere" option are different. If you view your security settings under System Preferences | Security & Privacy, you will see there is no Anywhere option. To enable the "Anywhere" option, run the following command in a terminal:

sudo spctl --master-disable

If you go back to System Preferences | Security & Privacy, you will see the Anywhere option enabled. Unlock the edit option by clicking on the padlock icon at the bottom of the window to change to Anywhere option. At this point, you should be able to install the software.

I am trying to install Thunderbird app on my iMac. I've downloaded the software twice, but each time I click on the app after downloading, I get a message that "Thunderbird app is damaged. Eject disk image." Any ideas?