Firefox seems to have some problems with captcha.
According to the following website, there seems to be a coding issue on the XN website that is subject to that glitch.
Copied from https://contactform7.com/faq/captcha-s-text-input-box-is-working-on-int…
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This is a well-known issue caused by misusing the <label> element. Make sure you are using it correctly.
This is good: