Create a new string (right click on top bar with fields name) widget.content.gtk-theme-override
And set it to Arc-Darker or Adwaita:light
Two Firefox profiles with different icons
Create two profiles (one as a default profile), you can use:
Go to about:profiles in Firefox.
Run firefox --ProfileManager --no-remote from terminal or ALT+F2
Remember names of your profiles (e.g. personal and business - I assume profile business is a default profile - it means that hyperlinks outside Firefox browser will open in this profile)
Create a personal desktop shortcut with different names and icons
Find an original Firefox desktop shortcut file: locate firefox.desktop - it should be something like /usr/share/applications/firefox.desktop
Create your personal profile desktop file: cp /usr/share/applications/firefox.desktop ~/.local/share/applications/firefox-personal.desktop
Change line Name=Firefox to something like Name=Firefox Personal
Change all lines with Exec=firefox ... to Exec=firefox --class=firefox-personal --no-remote -P personal ...
Change line Icon=firefox to something like:
Icon=firefox-trunk-alt
or any other icons from your icon theme find /usr/share/icons/ -iname "firefox*"
Business profile will be the default profile with default icon, name, etc.
Now you should see a two icons in your Gnome3 menu and two icons in your ALT+TAB and Dash panel (if you have it).