Perhaps it's a Firefox issue. Seems like I read/heard that different browsers can make the difference.
So, try using IE to see it it works from there.
If I was already in IE and having problems, I'd "delete files" on and offline under "Tools>Options" and try again. Also, be sure to type it in completely, not just let the drop down menu fill it in....
Also, yes, better to do it via cable (on the LAN side), in case wireless access to the router is turned off.
Another easy trick is to right click on the connection (in the task bar) and hit "repair" this flushes out various things that get clogged up in the connection. Type in the address again after reconnecting.
Finally, unplug and replug the router to see if that helps.
Default for this one is 192.168.1.1, right?
Steve