Rather than just opening, Firefox tries to be cute by opening the tab halfway, then sliding open the rest of the way. If you have a fast enough system, maybe you don’t notice. But if you’re like me and like things to be as fast as possible, you can disable this behavior.
In a new tab, open about:config. In the filter, type browser.tabs.animate. Double-click the value to toggle it to false. The change takes place immediately–there’s no need to restart Firefox. Opening tabs will then happen slightly faster, as will launching Firefox with multiple tabs open.