When enrolment keys are "un masked" in the course settings Firefox "remembers" keys from previous courses. This means: 1) Teachers issue the wrong keys to students 2) Teachers may inadvertently change the key if it is un masked and then saved There was another bug tracker relating to this issue MDL-9929 The solution was to rename the "password" field, however, I notice that Petr ?koda left a comment: > "rename the password field - not sure if it would not "reuse" passwords from other courses" Well... now we can be sure, that is exactly what is happening! I have read other comments related to other similar issues stating that this is not a Moodle issue but a FF problem, whilst I agree with that it does not solve the problem! Petr ?koda (in MDL-9929) mentioned another possible solution (hack): > "autocomplete="off" - brakes validity - IE invention, should work with FF too" Perhaps it would be worth trying that.