Chrome simply keeps running in background sometimes even when you close it. Go to task manager, switch to processes tab, and right click at every chrome.exe process and select end process.
This is where chrome sucks, if you are using firefox, you will see only one process called firefox.exe no matter how many firefox windows and tabs you've opened at a time. But chrome create each separate process for each new tab or window, it simply floods your process with multiple processes.
There are several reasons, one I've mentioned above. If you aren't gonna go deep into technical things, and if you don't care much about what is happening at the background in your system, then chrome is best for you since it offers best speed to users. But if you are concerned about these things, then firefox is the way to go.
Above all, FireFox is best browser for web developers and software engineers, who are going to do a lot of trouble shooting and debugging on websites. That is why I prefer firefox the most as a programmer.
