Boy this has to cover a lot of ground, the add-on architecture of Thunderbird is changing. 1. Addons.mozilla.org is for Firefox addons. There are no Thunderbird add-ons hosted there (esxcept for some dictionary files) 2. Addons.thunderbird.net (ATN) hosts all Thunderbird addons. It is best used through the addin manager of Thunderbird to search as it will offer the correct version of an addon for your version of Thunderbird (if there is one. This is going to be very important for the future while every version has different compatibility issues. Some addons like Lightning has many versions on ATN and selecting the best for your version of Thunderbird is best left to the computers. 3. Addons that work with Thunderbird 60 will not work with 66, they probably will fail in beta versions before 66. 4. The age old practice of finding an XPI file and trying to install it is not a good option at he moment and unlikely to be until the Thunderbird webapps API (much like the Firefox webapps but for mail) is finalized. Although it will probably have a name change before release as it will not be web focused but mail focused. 5 Basically in the version of Thunderbird you are using there are almost no add-ons that actually work and even fewer that are feature complete. Hence the hiding of the actual XPI files. They are no longer one size fits all. Far from it as the basics of loading an add-on are completely different in 66 than in 60. In the site addon.mozilla.org you can still right click the install in Firefox button and select save link as to save the underlying XPI file, but as it will only run in firefox there is little point beyond the ability make your own archive of installed add-ons for some reason.