This affects all Firefox keyboard shortcuts, such as Control+Tab for switching to the next tab. It also affects some keys that Windows normally handles, such as volume up, volume down, and mute. Fortunately, it doesn't affect keys handled by HoeKey, a hotkey program. This means the problem with volume keys can be fixed by handling volume keys via HoeKey, instead of directly through Windows. Here is what needs to be added to the HoeKey configuration file:
173=Msg|Progman|793|0|524288 ; mute
174=Msg|Progman|793|0|589824 ; vol down
175=Msg|Progman|793|0|655360 ; vol up
I expect other hotkey programs could do the same thing. I like HoeKey because it uses minimal system resources, it works perfectly, and is free.