How can one make Firefox ignore my GTK theme entirely?

Permanent solution:

You can specify a light GTK theme to use for rendering webpages in about:config. This is great because your system theme will still apply for menus etc. but the webpages will be rendered as they are with light themes.

1. Navigate to about:config in your Firefox browser
2. Right click and select new->string
3. Enter widget.content.gtk-theme-override as the name
4. Enter a light GTK theme that you have installed on your system for the value. e.g. Arc
5. Restart firefox.

Source: https://www.reddit.com/r/firefox/comments/6rbvvw/photon_rectangular_tabs_have_landed_in_nightly/dl4318f/

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1283086

Start Firefox with one of the following command:

• GTK3 Firefox 46+: env GTK_THEME=Adwaita:light firefox
• GTK2 Firefox <46: env GTK2_RC_FILES=/usr/share/themes/Simple/gtk-2.0/gtkrc firefox

This will launch Firefox using the Simple/Adwaita:light theme, which is pretty basic and should blend with your Persona. You can try other themes, by replacing Simple with the theme name, but this doesn't work with all themes.

So far I have successfully tested with Simple and Redmond themes...

Before

After

UPDATE: My original answer actually gets overwritten when Firefox updates.

My current (permanent) solution

Edit
~/.mozilla/firefox/abcdef.default/chrome/userContent.css
(where abcdef is a random string)

Add

input:not(.urlbar-input):not(.textbox-input):not(.form-control):not([type='checkbox']):not([type='radio']), textarea, select {
    -moz-appearance: none !important;
    background-color: #eee;
    color: #111;
}

Original answer

I found a solution to make Firefox completely ignore your GTK theme.

Edit /usr/share/applications/firefox.desktop and change

Exec=firefox %u

to

Exec=bash -c 'GTK_THEME=" " firefox %u'

When you (re)start FF it will only use the FF theme.

I would like to add to the great answer of @lovinglinux a way to make Firefox to load the theme when opened from a launcher in the taskbar instead of command line:

1. Copy the Firefox desktop file to your home directory.

   mkdir -p "/home/$USER/.local/share/applications/"
   cp -v "/usr/share/applications/firefox.desktop" \
     "/home/$USER/.local/share/applications/"
   

2. In the Exec line, replace the value after = with a command using the instructions provided by the mentioned @lovinglinux:

   bash -c 'GTK2_RC_FILES=/usr/share/themes/Redmond/gtk-2.0/gtkrc firefox %u'
   

And the result is this:

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:

GTK_THEME=Adwaita:light firefox

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:

browser.display.use_system_colors

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:

MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox

export MOZ_APP_LAUNCHER

and change it to this:

MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox
GTK_THEME=Adwaita:light

export MOZ_APP_LAUNCHER
export GTK_THEME

Finally, restart Firefox.

Hope this helps.

Please help me by voting this up.

Thank you :)

This add-on (suggested by koushik) works well for me.

I'm using Firefox 57 on Ubuntu MATE 17.10

There is a very simple way to get the default color scheme back for web pages while still keeping your GTK theme for Firefox elements. I know it's not exactly what was asked, but it may help people coming here from a Google search.

1. Navigate to about:config in your address bar.
2. Scroll down until you see the settings
   • browser.display.foreground_color
   • browser.diaplay.background_color
3. If those settings have been modified, right-click them and select reset.

That should be it, no restart necessary. This works with Firefox Quantum.

Settings in about:config

I'm using the Adwaita dark theme with Ubuntu 18. I'm using the default theme in firefox 63.0. The best solution in my case, with no side effects at all, was to force firefox to use a light theme (Adwaita in my case) for web content rendering only :

• Open about:config in the address bar.
• Create a new string entry : widget.content.gtk-theme-override ( right click > New > String ).
• Set the value to the light theme to use for content rendering (Adwaita in my case ).
• Restart firefox.

Change the GTK theme for content process only

You could start Firefox with a different user and the same profile. Copy your current profile (~/.mozilla/firefox/profile) to that different user's folder, chown it to the user, and then on your console run:

su newuser
firefox -profilemanager

This sounds a bit overkill, but you can put it into a script. Firefox will use the theme on the profile, but keep the unthemed GTK from the new user.

Also, this is the kind of stuff that makes it fun to use Firefox and Ubuntu :P

You can add another theme to Firefox that you like from Tools -> Addons -> Themes. Then just use that theme instead of the default one.