Some websites use customized Google search (powered by Google etc) on their pages. If you try to access search service on such sites using Mozilla Firefox having Adblock Plus plugin (ABP) installed, then sometimes you will not get proper searched results or it may return a blank page. This is because ABP blocks the (third party) Google scripts from loading on the page. To overcome that, you will have to "white-list" such scripts in ABP. Checkout this post for more on this.
PS: this happens with ABP 1.2 and FF 3.6.6.
