The best solution to tell Firefox that it should use the light theme instead of the dark is by specifying the GTK_THEME variable prior to launching.
So, 4 ways you get for doing this..
1- from terminal you can do:
2- if you hate the terminal and wanna close it once firefox works:
GTK_THEME=Adwaita:light firefox &
3- from firefox about:config you can set the following setting to false:
WON'T WORK FOR localhost SITES! so if you are a web developer/designer it won't work out that well for you.
4- if you want this to be the defualt behavior:
Just add two lines to the Firefox launcher script located at /usr/lib/firefox/firefox.sh
in the terminal type down:
sudo gedit /usr/lib/firefox/firefox.sh
look at the beginning of the script for this:
and change it to this:
Finally, restart Firefox.
Hope this helps.
Please help me by voting this up.
Thank you :)