A lot of work has been done to ensure that legacy extension (overlay extension and bootstrap extensions) will work in Thunderbird 68. Technically, they have to be converted to WebExtensions, but by activating the legacy mode, the general structure of the legacy extension does not need to be changed.
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 WebExtension 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.