RadRotator problem in firefox 3.6 - Horizontal line appears when we hover on left or right buttons [closed]

Question

i am using firefox 3.6 & vs2010 asp.net empty application with c#...

i do not know what is going on about my radrotator in firefox? (In IE Every thing Is Ok)

when we hover on left or right buttons In RadRotator , a Horizontal line appears!

how can i fix this problem?

thanks for attention