If you ever get stuck with a blinking cursor on a webpage in a strange spot that doesn't seem to be editable. You might have accidentally turned on Caret Browsing in Firefox. F7 is your fix.
From the dialogue box that is present by default from Mozilla Firefox:
Pressing F7 turns Caret Browsing on or off. This feature places a moveable cursor in web pages, allowing you to select text with the keyboard.