'git clone --recursive' failing

[happybeing]

Following the instructions in README.md the first step is failing (on Debian 8/Jesse):

$git clone --recursive  https://github.com/linkeddata/tabulator-firefox.git
Cloning into 'tabulator-firefox'...
remote: Counting objects: 9895, done.
remote: Total 9895 (delta 0), reused 0 (delta 0), pack-reused 9895
Receiving objects: 100% (9895/9895), 18.82 MiB | 1.27 MiB/s, done.
Resolving deltas: 100% (1467/1467), done.
Checking connectivity... done.
Submodule 'content' ([email protected]:linkeddata/tabulator.git) registered for path 'content'
Cloning into 'content'...
Warning: Permanently added the RSA host key for IP address '192.30.253.112' to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
Clone of '[email protected]:linkeddata/tabulator.git' into submodule path 'content' failed