Hi Evgenii, thank you for your review! I confirm there is a memory leak, it appears to be related to a known bug in Firefox https://bugzilla.mozilla.org/show_bug.cgi?id=1142446, which we updated today. We are developing a workaround now and I'll update my answer as soon as the new version is ready and reviewed by Mozilla.
Update Sep 18, 2015: We updated the add-on, now in version 1.0.17 it leaks significantly less than before. The leakage will fully disappear as soon the above bug is fixed.