I have verified that this issue is no longer reproducible with the latest Firefox Beta (86.0b4 Build ID - 20210131185630) an the latest Firefox Nightly (87.0a1 Build ID - 20210131211743) installed on Windows 10 x64, Windows 7 x64, and Windows 8.1 x32. Now, I can confirm the following that separator line's color on "Dark" theme is `#53515F`.
Bug 1689095 Comment 8 Edit History
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I have verified that this issue is no longer reproducible with the latest Firefox Beta (86.0b4 Build ID - 20210131185630) and the latest Firefox Nightly (87.0a1 Build ID - 20210131211743) installed on Windows 10 x64, Windows 7 x64, and Windows 8.1 x32. Now, I can confirm that the separator line's color on "Dark" theme is `#53515F`.