As you know, most add-ons are created by independent developers and not by Mozilla. They are not guaranteed to be useful or safe, although they do get some level of review. If you are referring to the keylogger, consider that you are installing a kind of spyware into your browser. If you lack confidence in what might happen to the information, do not install it! In his paper, he describes some proof-of-concept extensions showing how malware can exploit Firefox. This may well be one of those. To bring this to the attention of the reviewers at the Add-ons site, you might post over there on their forums.