I checked on Windows 10 x64, macOS 10.13 and Ubuntu 18.04 x64 using beta 67.0b18 and latest Nightly 68.0a1. I can't seem to reproduce the issue anymore. I think it's fixed.
Bug 1485893 Comment 14 Edit History
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I checked on Windows 10 x64, macOS 10.13 and Ubuntu 18.04 x64 using beta 67.0b18 and latest Nightly 68.0a1. I can't seem to reproduce the issue anymore. I think it's fixed. One thing I've noticed is that I can't seem to install in Nightly the add-on from comment 0. I used another one to test on Nightly (Best Homepage Ever - New Tab Quick Launch (Free)).