Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:66.0) Gecko/20100101 Firefox/66.0 ID:20190120213632
I have a MacBook with a retina display, which is usually connected to an external monitor with a resolution of 1680x1050. That external monitor is then used as the primary screen where Firefox runs on.
Whenever I disconnect the monitor or connect it again, the displayed content isn't adapted to the new resolution/dpi immediately, but with a delay of some seconds. Or if Firefox isn't the focused application it will never happen.
As attached you can see the situation when the external screen got connected. Firefox is not the focused application, and doesn't update the displayed content in tabs. But once I click somewhere in the tab and Firefox receives the focus, the resolution change will be adapted.
Similar to the chrome UI I would expect that this change will happen immediately, when the screen gets disconnected/connected.