The 2 reviews, posted 4 years ago already, seem fair, make a good point, the add-on is lacking an automatic insertion mode. Also if the code were available maybe somebody would be willing to improve it. As it is works with copy and paste (...) and also seems to be abandoned as it is flagged by Mozilla as not actively monitored, maybe is unsafe.

Does the job, but the thing I don't like, it's that you have to copy/paste the characters. Clicking would be far better. (And also maybe why not adding more character through a little customization)