create and save a file called /home/yourname/firefox.desktop
with contents changed from my pattern
where I show pathway to executable as /a/firefox/firefox....yours is likely to be /usr/bin/firefox/firefox
where I show pathway to icon as /a/firefox/browser/icons ....yours may be /usr/share/icons
so search for your icon and executable if they are different.
if you install packagesearch (a debian package) it will show you the full pathway to files without you having to learn the cli