Rated 5 out of 5 stars

Thunderbird 68.4.2 (32-bit) -- upgraded recently
Mail Redirect 0.10.6 -- upgraded recently

Invoking "Mail Redirect" with ctrl-B brings up a blank window -- the window doesn't have a resend-from selected, and the message that I'm trying to redirect isn't listed in the blank window. Is this a known upgrade problem?

[2/17] Resolved quickly! See developer's reply below. Love this add-on :-)

This review is for a previous version of the add-on (0.10.6). 

Something changed in Thunderbird that makes your default account invalid, and Mail Redirect can't handle that properly. If you go to Tools > Account Settings, select your primary account and choose Set As Default from the Account Actions button in the bottom left, your default account should be good again and Mail Redirect should work again.
Hope that helps,
Onno