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This is how you can make a so called global add-ons installation in Firefox and Thunderbird after version 3.54

You need to rename the extension to the GUID or email address as shown in the file install.rdf in the xpi file or what you see if you install the extension in Firefox.

DOM Inspector: inspector [AT] mozilla [DOT] org [DOT] xpi">inspector [AT] mozilla [DOT] org [DOT] xpi

Usually renaming the installation xpi file to email [AT] address [DOT] xpi (found inside the install.rdf file in the xpi file) is working just fine.
To open up the xpi file to find the e-mail address, jsut use any unzip software, Ie: 7zip
After you have renamed the file to email [AT] address [DOT] xpi all you need is to locate your global add-ons directory.
If the add-on have a " <em:id>{972ce4c6-7e08-4474-a285-3208198ce6fd}</em:id> " then that should be used instead of email [AT] address [DOT] xpi
Filename then would be {972ce4c6-7e08-4474-a285-3208198ce6fd}.xpi

<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id> dows not work since that is the id for Firefox

In Windows it's usually located in " C:\Program Files (x86)\Mozilla Firefox\extensions "

This information can also be used if you make a clean installation of Firefox and want to ahve your favourite add-ons installed at first start.
Or when you change computers.

Source of information: http://support.mozilla.org/en-US/questions/874958