Browser updates have security improvements and if you don't update you don't benefit from the changes putting your computer at risk for viruses or spoofs.
Updating your browser could make it faster! They are perfecting many of the tools for the internet.
So how do you know if your browser is up to date? Go to HELP on your browser and at the bottom is usually an "about" link which will tell you the version you are currently using. Some browsers will automatically update, but others don't so it is worth checking.