User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0
Steps to reproduce:
When changing DPI from 96 DPI to 144 DPI (1.5 scale factor), Firefox stays unaffected.
Firefox should scale to the new DPI setting. Currently, on Linux, Firefox is listening to "notify::scale-factor" event. However, scale factor is only an integer value. I think Firefox should also listen to "notify::gtk-xft-dpi" which is used to compute scale factor. This would be something like:
(but it would need another callback as it is not attached to the widget)