Well, I knew there had to be an easier solution, and I found it. Uninstall my extension, and add this to your userChrome.css file:

.tabbrowser-tab[selected="true"] { -moz-user-focus: normal !important; }

That should restore Firefox to it's prior behavior (from 3.0.*). For some reason it was set to 'ignore,' which according to MDC:
By setting this to 'ignore', you can disable focusing the element, which means that the user will not be able to activate the element.

For what reason, I have no idea.

Weird.

Just realized a flaw to the extension as well, you can't do ctrl+right/left commands in text boxes amongst words.

Ah, I thought there was something I was forgetting, I just couldn't remember what (if anything) Ctrl+Arrow was used for. I never use that for moving by words, but yeah, I wouldn't want that disabled. I guess I could have had it only move tabs if the selected element wasn't a textbox or text input.

But the userChrome.css solution is much better anyway. That way you're still using the built-in keypress event handler provided by Firefox.