/usr/share/mozilla/extensions/ is in xul-ext-useragentswitcher 0.7.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/mozilla/extensions/songbird@songbirdnest.com/
/usr/share/mozilla/extensions/songbird@songbirdnest.com/{e968fc70-8f95-4ab9-9e79-304de2a71ee1}
/usr/share/mozilla/extensions/{92650c4d-4b8e-4d2a-b7eb-24ecf4f6b63a}/
/usr/share/mozilla/extensions/{92650c4d-4b8e-4d2a-b7eb-24ecf4f6b63a}/{e968fc70-8f95-4ab9-9e79-304de2a71ee1}
/usr/share/mozilla/extensions/{a463f10c-3994-11da-9945-000d60ca027b}/
/usr/share/mozilla/extensions/{a463f10c-3994-11da-9945-000d60ca027b}/{e968fc70-8f95-4ab9-9e79-304de2a71ee1}
/usr/share/mozilla/extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}/
/usr/share/mozilla/extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}/{e968fc70-8f95-4ab9-9e79-304de2a71ee1}