Please read: Remember to return and click the Accept button ONLY once your issue is resolved. If the first answer does not work, just REPLY to me and we will continue.
This is a long time issue with Chrome, usually installing it again resolves it outright.
I've created a registry file for you that should resolve it. Close Thunderbird.
Download the file, double click it (or right click and click Merge). At the prompt, choose Yes.
This should re-write the proper entires that open HTML documents from email. Test it after with Thunderbird.
If it does not, please REPLY and let me know and I'll try to guide you through it manually.
I tried the fix, however it did not work and when I tested an email to the Internet and the blinking mouse showed up and my system stop working.
Found 1 virus (BadJoke.KillFiles). Now what?