How to Make About:Blank Page Black or Any Color in Firefox

I found out what was wrong. The saved as type was switched to *.* in the save file dialog (for the save as file option obviously). With this setting on, Firefox was saving the original html. With any of the other settings (including txt) Firefox was saving content from DOM (with all the changes). For a second I was wondering if this is perhaps an undocumented feature, but here it says:

All files: This is equivalent to "Web page, HTML only," but you may specify a file extension (e.g. ".htm" or ".shtml").

Which would makes me believe it's a bug. I mean, it's a bug because the behaviour differs from the "Web page, HTML only" option.

Update: I tested that yesterday and I was quite sure that Web page, HTML only saved a modified version of the html, yet when I repeated the test on another machine I got the original html file. So the "bug" part might actually be my mistake. Nonetheless, if your firefox saves you the original file instead of the modified content, then make sure that saved as type is not set to Web page, HTML only or All files.

I found an answer from http://forums.mozillazine.org/viewtopic.php?p=2989626#p2989626 with help from http://kb.mozillazine.org/index.php?title=UserChrome.css.

The Mozilla directory depends on your operating system:

Windows NT (NT4.x/2000/XP/Vista/7/8/10): "%APPDATA%\Mozilla\"

Windows 95 (without Desktop Update): C:\Windows\Mozilla

Windows 95 (with Desktop Update)/98/Me: C:\Windows\Application Data\Mozilla\, C:\Windows\Profiles\[Windows login/user name]\Application Data\Mozilla\

Unix/Linux: ~/.mozilla/

Mac OS X: ~/Library/Mozilla/, ~/Library/Application Support/

After finding the Mozilla directory you have to go through a few more directories to find your profile directory. See http://kb.mozillazine.org/Profile_folder_-_Firefox.

In your profile directory, if it doesn't exist, create the chrome directory. Then create a new text file in chrome called userChrome.css with the following contents:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
browser { background-color: #333 !important; }

You can change the background color to whatever you want. Unfortunately for me the screen still flashes white very briefly but this fix is still very nice.