LR.
Do you mean you downloaded it but FF complained when you tried to install/run it? Weird.
Netstumbler is just fine. I've used it for years. All it does is scan all wirelesss networks in range (including hidden ones) and will show you the channels they use and the received power (signal to noise ratio). By selecting a channel not in use and looking at the power you're receiving from your access point, you can often solve these problems. Bear in mind that it only shows WiFi so, for example, you don't know if you're getting interference from some other source. That's why picking unused channels and checking the power is the way to go.
You don't necessarily need an unused channel. I'm in an apartment block with perhaps 20 WiFis in range. I'm using channel 8 which is only used by one other network which I guess is far away as the power I get from it is tiny and therefore little interference.
Cheers