Not able to reproduce it because this tab is not loading with any webpage right now.
Couldn’t say how or when it happens because I’m not using this tab every day, but it seems not to be linked with the content of the page.
On my computer, both with a small script of an empty HTML5 doc or a Youtube full page, it’s not working.
I’m using Firefox Developer Edition : 67.0b14 (64 bits) on Windows 10 (x64) 1809 version.
Hope it helps