Unsteady Screen zooms in and out on it's own.

[p38p51]

Hello. I seem to often have troubles with my FR24 screen zooming in and out on it's own, particularly at beginning. It usually eventually settles down and is steady
but this often takes quite a long time. I am using an iMac OS10.8.5. Thanks for any assistance./Bill

[p38p51]

I was using Firefox Browser.