I have seen this particular issue for a long time. Basically when replying via the Rich Formatting option, the reply field will not word-wrap. It usually happens when quoting the port you are replying too. Not only does it not word-wrap, but it starts scrolling the page horizontally, truncating the page on both sides. The below image can describe it better than my clunky attempt:
Note that the image is not truncated on the sides. I didn't cut anything out. That is how it actually looks. It is quite frustrating.
I believe this may be specific to FireFox as I have not really experienced the issue when I used Chrome (the few times that I did use it).
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