Pdf.js (which Firefox uses) does support filling out Acrobat Forms (acroforms as well as XFA). Possibly the PDF you have is corrupted? You can read more about

Pdf.js (which Firefox uses) does support filling out Acrobat Forms (acroforms as well as XFA). Possibly the PDF you have is corrupted? You can read more about PDF.js support for forms here.[0] Libreoffice also supports creating Acrobat Forms.[1] [0] https://blog.mozilla.org/attack-and-defense/2021/10/14/implementing-form-filling-and-accessibility-in-the-firefox-pdf-viewer/ https://blog.mozilla.org/attack-and-defense/2021/10/14/imple... [1] https://www.linuxuprising.com/2019/02/how-to-create-fillable-pdf-forms-with.html https://www.linuxuprising.com/2019/02/how-to-create-fillable...