For version 0.5.0.0 or later:
Check the checkbox in the new supported site found prompt dialog.
For version prior to 0.5.0.0:
You can archive your requirement by making changes to two configuration items.
Tools->AutoPager->Manage Setting->General
Uncheck "show prompt for individual new site" and "show simple modal prompt for individual new site".

You can also use about:config to do this.
You need change these two perf:
set autopager.noprompt to true
set autopager.modalprompt to false