Why is Adblock Plus doesnt block advertisement in firefox while using an I Mac?

my fren she's uses an i mac . she browse using firefox. the other day she installed adblock plus. after computer restarted she went to firefox and clicks on youtube she finds that the ads there are not block. a check in preferences shows that it doesnt contain any filter . what is a filter?? and please help by... show more my fren she's uses an i mac . she browse using firefox. the other day she installed adblock plus. after computer restarted she went to firefox and clicks on youtube she finds that the ads there are not block. a check in preferences shows that it doesnt contain any filter . what is a filter?? and please help by showing where she can get some filter to be add in to the adblock plus.

thank you. thank you for ur help....
3 answers 3