You could’ve ignored 3D View feature aka Tilt in Mozilla browser, which is introduced with Firefox 11, though this feature is intended for developers to view their pages in 3-dimensional  mode; when you enter this mode, it allows you to analyze the structure and page elements in different angles.

 Firefox 3D View: View webpages in 3-dimensional mode

By not knowing the purpose novice users can play with this, but 3D View is very helpful for developers in finding layout problems and stray content on their pages. The developer can use this built-in standalone tool in Firefox to optimize the layout by looking at the page structure.

UPDATE June 13, 2016:

FYI, Tilt or 3D View doesn’t work in Firefox when e10s is enabled, that’s why Mozilla has removed the 3D View from Firefox 47. But the feature is available in the form of an add-on here, but for it to work, you need the browser to run in non-e10s mode.

3D view in non-e10s mode

1. To open 3D view mode when Firefox is open, press Ctrl+Shift+ I to open Web Developer Tools and click on ‘3D View’ button that is after the Paint brush (check the screenshot below).

Update: In Firefox 31 and later versions, Developer Tools doesn’t show 3D view button, no problem! it is still there. Click on Gear icon ( Toolbox Options),  scroll down, under ‘available Toolbox Buttons’, enable ‘3D View’, now you can see its button.

Firefox 3D View

Or Right click on the page and select ‘Inspect element’ to open the page inspector and then click on the 3D View button (can’t see this button? May be your graphics card driver needs an update).

2. To exit this mode, click on the same button again.