You posted from Win10, while I'm using Win7 and don't see that difference between my Firefox 55 installation and an older version that I also have installed.

It could be a difference between the default theme between Win10 and Win7, but in your screenshots I am seeing a bit of what looks like a faint graphic section that I wonder about the cause. Do you happen to be using a light-weight theme (called Personas years ago when they first appeared in Firefox) ?

the-edmeister said

You posted from Win10, while I'm using Win7 and don't see that difference between my Firefox 55 installation and an older version that I also have installed. It could be a difference between the default theme between Win10 and Win7, but in your screenshots I am seeing a bit of what looks like a faint graphic section that I wonder about the cause. Do you happen to be using a light-weight theme (called Personas years ago when they first appeared in Firefox) ?

Thanks for reply. Yes, I'm using light weight theme in Win 10. This one: https://addons.mozilla.org/en-US/firefox/addon/light-bubbles/

How about with that theme disabled?

If I disable it shows dark icon, I can't see different between 55 & 54, if I use official Dark theme, it shows like below picture. The point is if I use a theme which not very dark, the shadow will make white icons clearer and beautiful.