Unless you need to do something very specific, as mentioned by others there are better ways to automate HTTP requests than opening a browser.

Either way, you can automate most GUI programs by simulating keyboard presses and mouse clicks and taking screenshots to "look" at the screen. See Autoclicker! Cheating is winning, too! for an example of an autoclicker. X11::GUITest also supports keyboard input.

I actually use X11::GUITest as part of a commercial product (cash register). After a user logs in, the cash register opens a website with an HTML Canvas that needs to go fullscreen. But browsers don't allow JS to put an element into fullscreen mode without user interaction. So the website informs a locally running (minimal) tool that it wants to go fullscreen, and the local tool simulates a mouse click to circumvent browser "security"...