Rated 5 out of 5 stars

Manual fix for version 1.3.2:
Changing line 49 of content/browser.js (dictionary-switcher.jar) to
var spellchecker = Components.classes[spellclass].getService(Components.interfaces.mozISpellCheckingEngine);
seems to fix the problem.

Would be nice to see an official version 1.3.3 soon :-)

This review is for a previous version of the add-on (1.3.2.1-signed.1-signed). 

Rated 4 out of 5 stars

It is not a bug in the add-on itself which makes it not working properly anymore (unless you apply the tweaks mentioned in earlier comments).
As long as bug https://bugzilla.mozilla.org/show_bug.cgi?id=843727 is not fixed, this add-on will continue to have problems. The bug affects a core add-on interface that is out of control for the programmer(s) of dictionary switcher.

This review is for a previous version of the add-on (1.3.2.1-signed.1-signed).