I have Mac OS X 10.6.8, Firefox 5.0.1, and no add-ins. Zoom settings are normal, I reset just in case. I went to work and viewed my page on a brand new out of the box iMac with FF 5 that I had never used before and the white line was there. Unfortunately I have no explanation for why some can and some can't see the white line.
I took a glance at the /styles/blue/buttons.gif though. I saw the white all around the button. I filled it in with a similar blue so the whole button was solid blue and overwrote the default one, the white line was then gone. Could Firefox be drawing the button bigger than it is and some of the white is bleeding over?
Let me know if there's any way I can help
Thanks,
Bri