That is a change that was made in Firefox 56.0. The appearance may vary slightly on the various operating systems.

Strange thing is that it appears that your posted in this forum using Firefox 52. Or is that screenshot from another Firefox installation or maybe a different device?

Thank You!! Yup, I did actually ask this question from my work, where I use Firefox 52, but the screenshot was from my home computeres, where I have got the newest 56.

Thank you for you kind answer! Jan

If the color gets you down, you can use a custom style rule to color in the gray, but changing them to a different picture may be quite tricky. For example: All bookmarks have grey folders-update 56.