disappearing interface after waking up the computer

Ever since the release of Firefox 66.0.0 a strange bug appeared which Mozilla can't cofirm, which makes me think the problem is with FVD Speed Dial: when I wake the computer up and Firefox is running, the whole interface disappears and I have to restart the browser in order to fix it. I made 2 screenshots - one with the normal interface: https://i.postimg.cc/V6mVDkg5/normal-look.png and one with the missing interface: https://i.postimg.cc/HLQNwyQf/after-waking-up.png

At first I thought it was a Firefox bug considering the address bar and the whole browser interface disappeared. But neither Mozilla or any other users can cofirm it as a Firefox bug, so the only possible explanation is that the cause of it is FVD Speed Dial. I have reported it to Mozilla and now I'm reporting it to you, hoping that one of the two companies may fix it.

In case the operating system matters for the problem, it's Linux Mint 18.3 Cinnamon x64.