Xmarks is free (gratis) but not free as in freedom. If you check their
license
(https://addons.mozilla.org/en-US/firefox/addon/xmarks-sync/license/4.0.2) it
is not a free software license.
===From TOS===
Copyright and Software License
The software comprising the Xmarks Add-ons is copyright Xmarks 2009. The
content we originate on Xmarks.com or SearchTabs.com ("Our Website") is also
copyright Xmarks 2009. Xmarks grants you a non-transferable, non-exclusive,
non-sublicensable, royalty-free and fully paid, worldwide right and license
to reproduce (solely to install and execute) the Xmarks Add-ons, in the form
provided only, for your personal, non-commercial use only, subject to the
terms and conditions of these Terms. You are permitted to download the Xmarks
Add-ons on as many computers and as many times as you like, provided that you
agree to these Terms each time you download or install the Xmarks Add-on.
So that is why it isn't included.
