Can turn the Firefox Menu on by musing to top of Browser and Right Click and tick Menu Bar or far right 3 Bar Menu then Options. https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/

Please let us know if this solved your issue or if need further assistance.

Firefox 52+, except for the Extended Support Release, does not run the Adobe Acrobat plugin. The built-in PDF viewer probably does not support PDF forms (either correctly or at all).

Therefore, I suggest opening any such forms externally in the stand-alone Adobe Reader/Acrobat application. Is that possible? If it won't work outside of the browser, then you can use Internet Explorer.