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I checked the form and viewed it on a Firefox v41.0.2, unfortunately, I'm unable to replicate the problem.
Here's a screencast (click the image to expand):
May we know if the form is embedded on a website and what computer OS you are using? If the form is embedded in a website, you might need to use the iFrame embed code to avoid any code conflict between your website and the form's resources.
We'll wait for your response.
Pardon me for butting in, but is this related to my problem covered in the thread "Numeric field stopped accepting input"? If so, the same work around might work here.
Thanks for your insight John, I'm not sure though. The forms of the user does not have any number fields in them, so the workaround may not work. We'll still waiting for the additional information to better assess the problem.