The picture beyond Thunderbird version 60 is rather uncertain, given the continued high rate of change in Firefox platform code. We will work with add-on authors in the coming months to determine a viable post-60 add-on strategy. Here are some general thoughts.
* Work has been undertaken to keep supporting legacy add-ons "somewhat" in Thunderbird 63 and beyond. We implemented an overlay loader that will load XUL overlays for add-ons but may cause graphical glitches or malfunctions. We're tracking this in {{bug|1476259}}.
* Legacy To use this overlay loader, XUL overlay legacy add-ons must replace install.rdf file with a WebExtensions style manifest.json file, which includes the key "legacy" set to true. See Lightning as an example :<br />https://hg.mozilla.org/comm-central/rev/e81f4b59a00a7d3e18d50fd3851ecbd47762a186#l2.5.** <small>(Note: The In this changeset the @variables@ are build variables, add-ons need to use fixed strings)</small>.<br />** NoteImportant: This only holds for XUL Overlay legacy extensions. Bootstrapped legacy extensions add-ons continue to work without a manifest.json file and will in fact break if you add that file.
* The general idea for TB Add-on authors who want to make their add-ons compatible with Thunderbird version from 63 and beyond to version 68 ESR will be to make have three options:* Rely on the mentioned XUL overlay loader. This is only a short-term stopgap solution.* Make their add-ons restartless, although the replacement for restartless add-ons in TB 68 isn't yet clear, or to WebExtensions, where we still have to define the interface. Further reading on restartless add-ons:
** https://mail.mozilla.org/pipermail/firefox-dev/2018-June/006503.html
** https://mail.mozilla.org/pipermail/firefox-dev/2018-June/006507.html
** https://bugzilla.mozilla.org/show_bug.cgi?id=1413432#c7
* Convert their add-on to a WebExtension. This is currently not possible since Thunderbird doesn't fully support WebExtensions and mail specific interfaces have not been defined yet.
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