If you don’t feel comfortable with deactivating the synchronization of your bookmarks, and don’t want to activate the add-on every time you need it as well, the only other option is to manually prepend “about:reader?url=” to the current tab’s address.

Possible changes in the add-on include “fighting” the Firefox synchronization by deleting every duplicate entry of the bookmark and a creation of the bookmark “on demand” by clicking another button in the toolbar if it’s needed. The first change is a rather dirty hack and the second one—if it worked—seems rather complex given the simple way one can switch to the Reader View on his own (see above).