Windows 7 got pretty close to solving this problem. Winkey+Num makes the n-th window on the taskbar active. Since you can modify the order of the taskbar icons

Windows 7 got pretty close to solving this problem. Winkey+Num makes the n-th window on the taskbar active. Since you can modify the order of the taskbar icons, this solves the spatial memory vs. fast switching between two windows. Winkey+1 for Firefox, Winkey+2 for Twitter, Winkey+3 for something else. This combined with Alt+tab that cycles in the same taskbar order (as opposed to MRU) seems like the holy grail to me. The problem is that this would break the existing expectations of Alt+tab, which is why I fear it won't happen.