DO IT AT YOUR OWN RISK!
What is u-boot?
U-boot is a universal boot loader which starts when you plug in the power cable. It runs itself to check the board and make some initial configuration. After feeling everything’s right, it loads the main OS and start the system. More information can be found at: http://wiki.openwrt.org/doc/techref/bootloader/uboot
Which u-boot to install?
I don’t know what stock u-boot can do, but certainly not much. As a geek, sometimes it’s just easy to “soft brick” the router when I dig into configurations and settings: the hardware is good but the OS is out of connection or stop working. For WR703N (and some other TP-Link devices with AR9331 SOC) users, it lucky to have someone like misp to develop a web failsafe u-boot: http://code.google.com/p/wr703n-uboot-with-web-failsafe/ and some like pepe2k to modify and improve that up to a new level: https://forum.openwrt.org/viewtopic.php?id=43237. The latter one is the u-boot I recommend.
This u-boot allow you to flash new firmware, u-boot and art through web browsers (not only firefox) or other ways. More details can be found at the link above. I strongly recommend to read that before doing anything.