Where should extensions be placed to be included in a custom build of firefox?

The advice in the Mozilla Support article Deploying Firefox with extensions:

  • Create the directory distribution\extensions
  • Find the add-on ID for each extension that you wish to add to this directory
  • Either:
    • Add the .xpi files of the extensions to this directory, each renamed to the extension add-on ID, while keeping the .xpi extension, or
    • Create in extensions a directory named add-on ID and unzip the extension into it.