Interestingly, Safari on OS X seems to have this problem all the time, too. (That is, I copied and pasted some lines in Safari on OS X from a searchfox page into a Firefox Nightly textarea, and there were lines pasted in the textarea.)
Bug 1616104 Comment 1 Edit History
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Interestingly, Safari on OS X seems to have this problem all the time, too. (That is, I copied and pasted some lines in Safari on OS X from a searchfox page into a Firefox Nightly textarea, and there were line numbers pasted in the textarea.)