I tested this issue on Windows 10, Mac OS X 10.14 and Ubuntu 18.04 and I have different results: On Windows 10 x64 FF Beta 69.0b13 and FF release 68.0.1 the behaviour from the description it doesn't happen, Is the same behaviour as in Nightly 70.0a1(2019-08-14). On Mac OS X 10.14 and Ubuntu 18.04, FF Beta 69.0b13 and FF release 68.0.1 the behaviour from the description is still reproducible. On Nightly 70.0a1(2019-08-14) it doesn't reproduce. So to sum up, the issue is still happening on non-nightly versions on Mac and Ubuntu OSes.
Bug 1552463 Comment 8 Edit History
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I tested this issue on Windows 10, Mac OS X 10.14 and Ubuntu 18.04 and I have different results: On Windows 10 x64 FF Beta 69.0b13 and FF release 68.0.1 the behaviour from the description it doesn't happen, Is the same behaviour as in Nightly 70.0a1(2019-08-14). On Mac OS X 10.14 and Ubuntu 18.04, FF Beta 69.0b13 and FF release 68.0.1 the behaviour from the description is still reproducible. On Nightly 70.0a1(2019-08-14) it doesn't reproduce. So to sum up, the issue is still happening on non-nightly versions on Mac and Ubuntu OSes but not on Windows.