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
Go into the advanced preferences editor, create a boolean preference called
nglayout.debug.disable_xul_cache if it doesn't already exist, set it to true, and restart Thunderbird.
DMO claims that specifying the
One way to make this happen automatically is by installing the userChromeJS add-on and then putting the code above into the file
chrome/userChrome.js within your Thunderbird profile directory.