Have installed the Firefox Web browser and placed it in the side menu. But only a white square with a red cross shows up and not the Firefox icon?

Usually a white box and + symbol means the system can't find the specified icon. typo? Did you install the CR version from the metapackage installer or from the download at mozilla.com As far as I can tell you need to create your own .desktop file when you do the download from mozilla method.

as an alternate, you can change the icon for the launcher on the side bar by right clicking>properties>edit items> and select a new icon.