Maybe you are in a rather busy area, as lots of stuff is in the 2,4 GHz range. Are you running Vista RC something? That had limited power enabled for WLAN. There are also software settings that regulate the WLAN signal power. If your software doesn't let you do that, try in the hardware manager under system.
Maybe your Laptop goes into power savings mode with everything when it runs on battery only.
May sound stupid, but -> change the channel. My router has 12 settings for that, but not all cards seem to receive all 12.
What kinda line are you on? Maybe the server was just busy the time you tried with the laptop. Go to the fastest site you are on and try multiple times with big files(gamershell, or your provider). There are things like wmm, try it all. Are your neighbors using WLAN? Have you secured it? Tons of stuff... You gotta give us something here, come back when you've tried everything, or know more about your problem.
As the last resort, have a mate with a laptop come over and test it.
//edit: Speed up firefox? WTF? I never did anything like that, and I constantly max out my 6 Mbit line via WLAN, and I never have to wait for a site to render/load even if I dl stuff @max... don't bother with shit like that...
