Rated 4 out of 5 stars
I agree with Sergio3N. (Win7, TB 78.3.2, 64-bit)
As a matter of fact, the extension seems really to be deactivated: the options page in the Tools menu is inaccessible too.
I know what's the problem: QuickText is in conflict with QNote (https://addons.thunderbird.net/thunderbird/addon/qnote) that Sergio3N is using too but I don't know which is responsible!