Apparently there is a problem for some people (myself included) where Autohotkey (AHK) scripts aren't working in Firefox. I have a "master" AHK script which does many things, for instance it keeps my middle mouse button from opening multiple copies of a tab from a single click, and it also remaps the "Capslock" key to act like the "Backspace" key (saves my right pinky some stress while touch typing).
I found a few mentions of it in message boards, but no solutions, besides reverting to an older version of Firefox.
I'm running Firefox version 46.0.1 and Windows 10, and I solved this problem by running the script as an Administrator. I had that option available in the right click menu. If you don't, or want something more permanent than right-clicking each time to launch the script, compiling it to an exe file and then setting it to always run as Administrator (under "Properties", then go to the "Compatibility" tab) also worked for me. I'll launch the script directly, because I'm always tweaking and adding to it.
I'd be curious to see if running Firefox in Windows 7 compatibility mode would also work.
(Hmm, I also just checked and Firefox's shortcut was set up to always run as Administrator, not sure if that's default or something I did when troubleshooting something else. Worth checking, for anybody out there with the same problem. I don't want to close Firefox and restart out of Admin mode right now to test it but it's reasonable to assume that could be causing the issue.)