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.container {
display: flex;
justify-content: flex-start;
}
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If you're using (S)CSS or SASS, you have to write flex-start or flex-end instead of start or end respectively.
Like this:
justify-content: flex-end;
align-items: flex-end;
Similar issue on github for mozilla: https://github.com/mozilla/addons-frontend/issues/7312