It is stored in the registry somewhere, and it's different across keyboards from my experience. It would be easiest to just install AutoHotkey and use this:
SC##::Run, C:\Program Files\Mozille Firefox\Firefox.exe
You will need to replace ## with the scan code of the key and the path with your Firefox path. You can find the scancode of the key with this script:
SetFormat, Integer, Hex
Gui +ToolWindow -SysMenu +AlwaysOnTop
Gui, Font, s14 Bold, Arial
Gui, Add, Text, w100 h33 vSC 0x201 +Border, {SC000}
Gui, Show,, % "// ScanCode //////////"
Loop 9
OnMessage( 255+A_Index, "ScanCode" ) ; 0x100 to 0x108
Return
ScanCode( wParam, lParam ) {
Clipboard := "SC" SubStr((((lParam>>16) & 0xFF)+0xF000),-2)
GuiControl,, SC, %Clipboard%
}
simply run the script, then press the web browser hotkey to find the scan code. Change SC## in the first script to this scan code, and you're good to go.