It’s difficult to say for sure what the future will hold for Thunderbird extensions. A lot of work has been done to ensure that legacy extension (overlay extension and bootstrap extensions) will work in Thunderbird 68. Beyond that, we just don’t know.
Thunderbird is based on the Firefox code, and as they are changing things all over the place, legacy add-ons will require constant adjustments and certain functionality will probably be removed at some point.
Overlay extensions are problematic because so much of what they depended on no longer exists. Bootstrapped extensions are less of a problem but are still considered at-risk.
There are two types of changes which are required to make your legacy extensions compatible with Thunderbird 68+:
The legacy extension must be converted to a MailExtension by replacing the old
install.rdf by a
All legacy extensions need to be updated to reflect changes in Thunderbird core, like renamed/replaced API calls, removed support for some XUL elements (need to use HTML elements now) and much more.
After updating your add-on for Thunderbird 68 and beyond, it will be treated as a WebExtension. While uploading it to addons.thunderbird.net (ATN), you will see a warning, that you cannot upload a new legacy version of your add-on afterwards. However:
Let's assume the last published legacy version of your add-on for Thunderbird 60 is
2.6. Upload the new WebExtension version for Thunderbird 68+ as
3.0 and increase the version number with each new release of the WebExtension as usual. Every time you update the legacy version for Thunderbird 60, pick a version number from the
Get the release candidate of TB68, to test your updated add-on with the next ESR release of Thunderbird.