Is it still possible to configure userChrome.css to have the URL field in the Firefox 'Add bookmark' window?

Yes, I believe so.

Also, is userChrome.css configuration going to be used no more in future releases? If yes, then how will we be able to customize, for example, fonts in Library, side bar and context menu?

No.

.editBMPanel_locationRow[hidden] {
  display: unset !important;
}

deleted

.editBMPanel_locationRow[hidden] {
  display: unset !important;
}

It did not work. Is there anything wrong with my userChrome.css file?

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

.urlbar-input-box, .findbar-textbox, .searchbar-textbox {

font-size: 12pt !important;
font-family: "DejaVu Sans Mono" !important;}

#contentAreaContextMenu :is(menu,menuitem){
 font-size: 12pt !important;

}

#places * { font-size:16px !important; }

#placesList treechildren::-moz-tree-cell-text {
 font-size: 16px !important;

}

#placesList treechildren::-moz-tree-row {
 max-height: 18px !important;
 padding-bottom: 0px !important;
 margin-bottom: -2px !important

}

#bookmarks-view

{

   font-size: 20px !important;

}

.editBMPanel_locationRow[hidden] {

 display: unset !important;

}

Try to leave out the @namespace line as more and more content is in HTML namespace.

I assume that the other code in userChrome.css is working ?

Try to leave out the @namespace line as more and more content is in HTML namespace.

Now it's working, thank you so much.

I assume that the other code in userChrome.css is working ?

Yes, all the other code is working, with and without namespace above it.

If userChrome.css is "legacy", is there a "current" way to configure Firefox? I hope they are not going to remove these configuration options from the application!