How do I open this in thunder bird to read it?
You should be viewing it in the browser where you installed FlowCrypt.
For Thunderbird, there is a plugin called Enigmail that can be set up to work alongside FlowCrypt to view these emails, but may be a lot of work to get working.
We also plan to release a FlowCrypt plugin for Thunderbird in the future.
In general, FlowCrypt and Enigmail are compatible, so if your recipients already use Thunderbird+Enigmail for PGP encryption, you should be able to communicate with them smoothly.