Light is a fork of Firefox where many components (developer tools, less common APIs) have been "slimmed down" or removed entirely to cut load times and improve performance.
You can run the program alongside an existing Firefox installation. We noticed no problems after doing so, although it was slightly annoying that Light has the same icon as Firefox, perhaps confusing if you pin both of them to your taskbar.
Our first tests suggested Light really does launch much faster than the regular browser. We saw startup times fall by perhaps 40% on average, well worth having.
Initial RAM use also dropped by 20%, although that advantage didn't always seem to remain if we opened multiple tabs; sometimes Light appeared to use less RAM than Firefox, sometimes more.
The browser isn't just about performance tweaks, though. It also supports one or two extras, including "portable mode", where Light can use an INI file rather than Registry settings to find its user and temporary folders.