Do they work if you create an /distribution/bundles/ folder in the Firefox program folder?

You need to create a folder with the name of the GUID of the language pack and extract the content of the XPI files to the /distribution/bundles/ folder to make it work.

I tested it on Linux and it seems to work.
They do not show in about:addons, but if I set the general.useragent.locale pref properly and restart Firefox then the language changes.

• http://mike.kaply.com/2013/02/11/add-ons-in-the-distribution-directory/

EDIT: Looks that this only works with the en-US version

Thanks for the quick answer.

I tried your solution, I created the folder :

"C:\Program Files (x86)\Mozilla Firefox\distribution\bundles\{ec8030f7-c20a-464f-9b0e-13a3a9e97384}"

I also tried :

C:\Program Files (x86)\Mozilla Firefox\distribution\bundles\Langpack-en-GB@firefox.mozilla.org

It did not work, unzipping or not the .xpi did not seems to make much difference either. I did the test with an existing profile and a new one. Is there a parameter or a command line to automatically activate addons ?

EDIT : I stand corrected, it does work but only with the US version of mozilla. I was doing the test on my french mozilla version at home. I did a reinstallation with the US version that we uses for our VM and it worked just like you said. Thanks a lot.