Firefox is notorious for that on OS X.
You say you bought the router used - I assume you did a factory reset to clear out all the previous settings? If you didn't, that would be a good idea to make sure there aren't any previous settings that might be messing things up.
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I'll admit that I'm having a hard time deciding between a 6th gen Airport Extreme and the Asus U68.
I currently have the 5th gen AE now, and it's okay, but I need the faster local wireless network speed. I like nice looking gear in general, and especially since it's in my living room, and the AE is simple and clean and white which will blend into my white bookshelves. And it gets excellent reviews as an all-around great router, though not really aimed at "power" users. Frankly, I don't find it any "easier" to use than a "regular" router (if it was any router other than Apple's, I would never buy a router that required special software to access), and sometimes I get frustrated with the way the Airport Utility works. Just let me edit settings in the browser.
The U68 is garish by comparison, IMO. The photos make it look like the front of it is black "plaid" - does it really look like that in person? And all the antennas sticking up. However, the stellar reviews and outstanding performance appeal to my "I want the bestest fastest" side.
I'll probably end up getting the AE just because of its looks. I'm so shallow. 😱