Firefox: By default, Firefox shows just a blank white page when you open a new tab. If you'd rather have it open a custom URL or load what's in your clipboard memory, NewTabURL can do just that.
The extension consists of just a single preference pane, where you can set whether a new tab loads a blank page, the same page as you last had open, or a custom URL location. If you'd like to have Firefox load up whatever URL is in your copy-and-paste memory, NewTabURL can do that, too, and do it only once if you open a bunch of new tabs. A final, handy feature is automatic highlighting of the address bar when you open a new tab, so you can quickly get to where you're going after opening a tab to do so.
NewTabURL is a free download, works wherever Firefox does. The developer's page, linked below, has a more recent version than the add-on's Mozilla page, but you won't receive automatic updates that way, so it's your choice.