Firefox: deestroy if only disconnected
Firefox: deestroy if only disconnected

currently,
the whole content script is destroyed once the extension gets disabled,
preventing Vimium C from doing cleaning.

According to
https://stackoverflow.com/questions/38645274/firefox-webextension-how-do-i-run-code-prior-to-disable-uninstall/38647164
, there's no good&simple solution.

In https://bugzilla.mozilla.org/show_bug.cgi?id=1223425,
someone says a "disconnect" event listener may be allowed.

So make this commit.

Tests on FF66: the listener "may" be called, in a quite low probability.

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus