When I paste some text in Firefox into editor, it is white and is background. Some text can be above the pasted text, but Firefox allow pick out bugged text. Please, see the screenshot: File:New Visual Editor bug in Firefox.png.
Topic on 2017 wikitext editor/Feedback
Does this happen if you "Paste and Match Style"?
I don't know. Just see my step-by-step reproduction: File:New Visual Editor bug in Firefox (step 1).png → File:New Visual Editor bug in Firefox (step 2).png → File:New Visual Editor bug in Firefox (step 3).png. If you talking about Ctrl+Shift+V: alt step.
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I'm so sorry. This bug is only on my profile. I guess, I have some breaking extensions.