I have not been able to configure this with the Flirc yet. I can only confirm that "browser_back" is the correct key. I've been trying to get the Flirc command line utility under Ubuntu to work, but it doesn't seem to be working. That is, I can't seem to get the command line utility to work with any key, not just "browser_back". As far as I can see, the command line utility is required for learning browser_back.
I will try this again under Windows. I just haven't had the chance yet, as things have been a little busy lately. If it works, I will report back here.