I'm trying to install Enigmail 1.8.2 for all users. I thought I read somewhere that you don't need to unpack the XPI file. Whilst writing this, I did some experimenting and found a solution, but it's messy (see below). Is there an easier way? Fresh install of Thunderbird 31.6 on Windows 7 x64. Have observed the following: 1. If I install the extension for one user via the Add-Ons GUI (from the marketplace, not from a local file), it does work. The Appdata\Roaming\Mozilla\Extensions folder is empty. Folder {847b3a00-7ab1-11d4-8f02-006008948af5} is created in Appdata\Roaming\Thunderbird\Profiles\xxxxxxxx.default\extensions\staged\ and moved from staged to extensions when TB is restarted. 2. If I download the XPI file from Enigmail site and copy it to Program Files(x86)\Mozilla Thunderbird\Extensions folder, then Thunderbird's Extensions screen says "You don't have any add-ons of this type installed". 3a. If I rename the xpi to zip (e.g. enigmail-1.8.2-tb+sm.xpi.zip) then I can open it and see that the contents are similar to what was created at point (1). If I unzip the contents, then Thunderbird doesn't recognise the add-on, unless I rename the folder to the GUID above (which could be found in install.rdf or chrome.manifest, once you know what you're looking for). Presumably I could also have done a per-user install, copied the folder from the profile, removed the extension, then pasted the copy into the program folder. That would save looking up the GUID but is only a little less fiddly. 3b. After renaming the folder and restarting TB, it gives me a new tab "Install Add-on" which says "Another program on your computer would like to modify Thunderbird with the following add-on: ... Location: C:\Program Files (x86)". Once I allow the add-on, it is listed in Profiles\xxxxxxxx.default\extensions.ini. Aside: Disabling the add-on simply removes the line from extensions.ini, so how does TB distinguish between an unapproved add-on and a disabled one? It seems to be in extensions.json.