Mozilla has been on a quest to slim down Firefox. Apart from the memory consumption issues, where Firefox is seeing great progress, there's also the swiftness issue, i.e. how fast the browser is or, more importantly, feels to be to the users.
Firefox is getting better at this, but it's still behind Chrome in this respect, something that can't be said when it comes to memory consumption.
One particular area of focus is the newly introduced new tab page. The page has been somewhat controversial, but it's a step forward for Firefox. The page is not overly useful, it's just a list of frequently visited or pinned sites, but it's better than nothing.
However, this also means that there's now an apparent delay and slowness when opening a new tab, arguably one of the most frequent browser actions.
This is because the Firefox new tab page is in essence a web page that needs to be parsed and rendered. It's lightweight and Firefox is fast enough, but it's still not instant. Especially on slower computers, there's a noticeable delay between the new tab popping up and the new tab page finishing to load.
Thankfully, Mozilla is on it. Mozilla's Tim Taubert has been working on both "tricking" users into believing the page is faster and on actually making it faster. For one, many of the "loading" animations have been removed.
These serve a purpose in most cases, but here they just made the users aware that something was "loading" making the page seem slower than it was or at least making its slowness more noticeable.
As of Firefox 17, the current development version, Mozilla has "removed loading indicators for newly opened tabs. No spinning throbber, no flickering tab label. It only is a very subtle change but the whole action of opening a new tab feels a lot smoother again."
But that's just half the story. Taubert also made the page load faster. Optimizing the page's code or Firefox rendering weren't viable solutions, obviously, so, instead, Firefox will simply pre-load the new tab page and keep it in memory all the time.
This way, when users open a new tab, the page is already loaded and all Firefox needs to do is display it. The page is fairly simple and the memory hit this makes is minimal. The benefits though are near-instant loads of the new tab page.