I reboot all the time and that didn't change anything, but I just tried disabling all my extensions on Firefox and that worked, even without rebooting after. I even turned them back on one by one and tried after each re-enabling, and the issue didn't return. Even though I DIDN'T disable anything on Chrome, the form works on Chrome too now.

I had:
Connective signing extension
eID Belgium
Facebook container
I don't care about cookies
New XKit