After last firefox-update some of css3-code has been broken... Example (jsfiddle).
Display: flex;
only needs to be on the container that needs to be flexible, not the inner elements. Here's the updated CSS, if you need a specific width, you can set that on form {}
.
CSS
form {}
#flex {
display: flex;
border: 1px solid green;
outline: 1px solid red;
}
#flex > * {
flex: 1;
}
label {}
input {}