VimFx has a toolbar button. It looks like this: 
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If you focus a text input, the icon is greyed out. This is to show that your
keypresses will be passed into the text input rather than activating VimFx
commands. (Except some commands: See the <force> special key.)
When in Normal mode, clicking it opens VimFx’s Keyboard Shortcuts help dialog,
just like the ? shortcut does.
In other modes, clicking it returns you to normal mode. If you feel like VimFx does not respond to any of your keypresses, it might be because you’re not in Normal mode. If you don’t know how to exit that mode (or have accidentally removed the keyboard shortcut to do so), clicking the button is the way to “escape” back to Normal mode. (On such occasions, you can hover the button and its tooltip will say what mode you’re currently in.)
In Ignore mode, the icon also turns red, like this: 
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(Blacklisted sites enter Ignore mode automatically.) The reason is that the
whole point of Ignore mode is to “ignore” all of VimFx’s commands, passing your
keypresses to the page instead. It is helpful to know that you’re really in
Ignore mode, so you can be confident that your keypresses do what you expect
them to. (See Styling if you’d like to highlight Ignore mode, or any mode for
that matter, some other way.)
Other modes are easily detectable. You know that you’re in Find mode if the find bar is focused. If there are hint markers visible for the page, you’re in Hints mode. Finally, if there’s a blinking caret outside a text input, you’re in Caret mode.
A quick takeaway from this part of the documentation: If you don’t know what’s going on, look for VimFx’s toolbar button. It can often help you.