Did the OP look into Thunderbird's settings for this:
https://support.mozilla.org/en-US/kb/make-thunderbird-default-mail-client#https://support.mozilla.org/en-US/kb/make-thunderbird-default-mail-client#Force Thunderbird to check if it is the default
Thunderbird can be configured to check if it is the operating system's default mail client each time it is started.
At the top of the Thunderbird window, click the Tools menu and select Options, or click the menu button and choose Options.
On the Advanced panel, select the General tab.
Make sure that "Always check to see if Thunderbird is the default mail client on startup" is checked.
Click Check Now to check if Thunderbird is set as the default mail client immediately.