At first, thank you for your review. I'm really happy that you found it a great extension as an alternative to Zen's one.
Right now I'm a little busy, but I'll continue developing the extension soon. And at the same time, I'm waiting for the webExtensions APIs to be more mature too.
My chosen shortcut key (either Ctrl + Q or Ctrl + Shift + Q) is actually the shortcut for closing the window on many platforms. And there isn't an option for users to customize the shortcut keys on Firefox (Chrome browser has it), so I have to remove it for now.
Also why are you still using FF52? Even if you're using ESR, I think it's reached EOL now. And in many old Firefox versions, even pre-quantum or 57+ are full of webExtensions bugs and missing many APIs. I think you should at least switch to Firefox 60 ESR. Some information: the sidebar support for webextensions starts at Firefox 54.