In the file chrome/locale/de/firegestures/prefs.dtd the entity: swipe.continuous is missing.
As a result it is not possible to open the preference dialog.
It's sure that the source in the repository is missing the entity.
However, the XPI file on addons.mozilla.org site contains the entity as English one.
In the file
chrome/locale/de/firegestures/prefs.dtd
the entity:swipe.continuous
is missing. As a result it is not possible to open the preference dialog.