Abrowser and IceCat developer console prompt doesn't clear when hitting enter

In Abrowser and IceCat, the developer console prompt area doesn't clear when hitting enter; it evaluates the query, but inserts a newline into the prompt area instead of clearing it.

This was previously a problem in Parabola's Iceweasel; ping Emulatorman on #parabola on Freenode to ask how he fixed it.