Firstly just a heads-up on FireFox V49.0.2: it does not show the "I am not a robot" radio buttons at all in this browser. IE must be used.
Now, I have V11.xx installed and downloading the V12 BETA several times from different pages on your web site, it insists on only installing V11 again. I used 3 different browsers to check the download was OK. I uninstalled V11 and rebooted but it again insisted on installing V11. Coincidentally I have downloaded and installed the V12 BETA in a VM I have running for testing.
Also thank you for providing feedback on the error when using Firefox V49.0.2. To assist us and all other users, are you able to provide a screenshot of the error you described so we can determine where the issue lies with this browser?
On the homepage you will see the option to download TeamViewer 12 Beta as pictured below.
This should install version 12 Beta for you. If Version 11 is still being installed after following this path, we will ask that you submit a ticket so that our technical support team can assist further.