That's annoying. The keyboard shortcut for the inspector is Ctrl+Shift+i so Firefox may be somewhat confused about the signals from your keyboard if it thinks you are pressing 3 keys when you are only pressing 1.

Sometimes pressing and releasing the Ctrl key several times will resolve this issue, other times shutting down and restarting Windows will fix it. In other cases, the problem may persist because of an incompatibility between Firefox and custom keyboard software, but if only one key is affected, that doesn't seem so likely... We've also heard of low battery levels in wireless keyboards causing issues with wrong keyboard data...

thank you very much !