E-mail field doesn't indicate underline mark

When I try to type "_" the symbol gets blank.

Can you help me please?

When I check it on one of my test forms, I am unable to replicate the issue. 

I cleared your form's cache. I would like to recommend you to clear your browser's cache and try it again: How to refresh your browser's cache

If your problem persists, please give us further details about your problem. A screenshot would be great: How to Post Screenshots to Our Support Forum

maybe because I used Mozila instead of Chrome

I tried it both on the latest version of the Firefox and also on the very old version of Firefox. However, I am still unable to replicate the issue. Would you please let us know the form which you have the issue so that we can further check?

Please check my results: