I encountered a...glitch where I seemed to have changed my keyboard language in Notepad++. I fixed the issue with this Q&A. How do I prevent this from happening again?
As I was typing the above, I realized that I still had the problem in a form: it was not fixed in Firefox. Apparently, I had not just changed my keyboard language in Notepad++, but in my system in general. While Ctrl+Shift worked in Notepad++, I had to use Alt+Shift to fix it in Firefox.
I found out a lot more about this issue at http://www.daniweb.com/hardware-and-software/pc-hardware/usb-devices-and-other-peripherals/threads/94301/help-my-keyboard-does-french-instead-of-question-marks, including how to disable the accidentally triggered hotkey that altered the keyboard language.
There was a (default) hotkey combo set when I installed Windows 7. I disabled it by going to
Control Panel -> Region and Language -> Keyboards and Languages
click on Change Keyboards...
select the Advanced Key Settings tab
You can set or disable the hotkeys here.