Firefox Mobile is not supported by our controls: http://www.telerik.com/aspnet-ajax/tech-sheets/browser-support and we cannot guarantee there will be no issues with it.
Most likely, it simply takes more time to redraw the page when you page. This can be due to, for example, a slower device or the general browser performance. Also, full postbacks are more likely to show a flicker while the browser receives content and renders it, so I can suggest you use AJAX when paging.
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