The community member who posted the comment explains that while published sites work on Firefox, the builder does not. They want to use the most modern features and avoid wasting time fixing cross-browser bugs, so they will only support Chrome for now to move faster. There is no explicitly marked answer to the original post's question about why WS does not work on Firefox.
Published sites does, only builder doesn't. We want to be able to use all most modern features and we don't want to waste time fixing cross browser bugs.
