Colibri form layout broken on Firefox

Some forms are broken on Firefox, with fields appearing at the bottom of the page, see screenshot of the Wiki Admin > Export form.

Changing colibri.css as follows fixes the issue (99% replaced by 95%):

.xform input[type="text"], .xform input[type="password"] {
  width: 95%;
}


.xform select, .xform textarea {
  width: 95%;
}

Using Firefox Zoom (Ctrl-+), the layout breaks again.