I ran into an issue (yet again 😅) where my flex items weren't shrinking below a certain point in one browser but not another (e.g. it worked in Chrome but not Firefox). Turns out, it's a one-line fix: add min-width: 0; to the item.

.flex-container {
    display: flex;
}

.flex-item {
    flex: 1 1 0;
    min-width: 0;
}