For addons, please contact the developer directly, as Thunderbird staff is not involved with addons.

See https://addons.thunderbird.net/en-US/thunderbird/addon/quicktext/reviews/1176319/

When I follow the directions, Firefox is blocking the download of the xpi file recommended. Workarounds?

Well, you could close Firefox and download from Thunderbird.

See https://support.mozilla.org/en-US/questions/1521896#answer-1747664

FYI the delay has been that, for various reasons, Quicktext is being rewritten. The most important reason is to change it to pure Webextensions (well documented by our excellent add-ons support developer), which means it will be far less sensitive to Thunderbird version changes.

The author informs me that once this rewrite is complete you and others can more easily contribute code. For now, the best way to help is test quicktext v6 using the instructions at https://github.com/jobisoft/quicktext/issues/439.

You can get the latest prerelease of version v6.3.6 out today at https://github.com/jobisoft/quicktext/releases/tag/v6.3.6. You can download the xpi file, and in Thunderbird use Add-ons, click the gear icon, and choose "Install from file" to pick the downloaded xpi file.

Visit https://github.com/jobisoft/quicktext/releases and under the newest pre-release expand the Assets item. There you will see the latest quicktext_version.xpi file. Right+click on that xpi file to download it. In Thunderbird visit Add-ons, click the gear icon, and choose "Install from file" to pick the downloaded xpi file.