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For me it worked fine too till FF45 Update came out. Could the author please patch this so it works again?

Ta opinia jest o wcześniejszej wersji dodatku (1.7.5.1-signed). 

This bug should be fixed in 1.7.6 version, released now.
A workaround is to set the preference "extensions.profileswitcher.use_onbeforeunload" to false, but I suggest to try before the new version.