You can also use `H` for back and `o` for opening the URL prompt, among others. That requires you to be in the insert mode for typing in input boxes or interacting with the website you're on via keyboard shortcuts though.
I haven't used vimperator for a long time, but in petadactyl you can use C-o and C-i to go back and forth, respectively. The key bind matches pretty well to what it is in Vim too.