Apple has introduced a security change to Mac OS X 10.9.5 which causes the error message "Thunderbird" can't be opened because the identity of the developer cannot be confirmed to appear when you try to install Thunderbird on your Mac.

Here are some steps you can take if you see this message:

Option 1 - Recommended

1. Hold down the Control key while clicking on the Thunderbird file.
2. Click Open in the context menu.
3. Click Open in the following dialog window.

Option 2 - Open Thunderbird through the System Preferences

1. Click on the Apple icon at the top left corner of your screen and open System Preferences….
2. Click on Security & Privacy, then the General tab.
3. You'll see a message stating that Thunderbird was blocked. Click on Open Anyway: