Ytdyklly

Is there a built-in way in Firefox to blacken or to colour the about:blank page?

Will I maybe save some energy by doing so?

Yes you can change the color. As explained here you should

1. 
   

   Windows: go to %appdata%/mozilla/firefox/profiles and open the folder in there.

   
   
   

   On Linux the equivalent is ~/.mozilla/firefox/RANDOM_TEXT.default/

   
   
   

   On OSX it is /Library/Application Support/Firefox/Profiles/RANDOM_TEXT.default

   
   


2. 
   

   look for the chrome folder. In that folder, find userContent-example.css and rename it to userContent.css.

   
   
   

   The folder and file may not be there yet with newer Firefox versions. If so, simply create them manually.

   
   


3. 
   

   Add this line to the file

   
   
   

   @-moz-document url-prefix(about:blank) {*{background-color:#000000;}}
   
   

   
   

As to whether it will save energy, probably not:

On LCD displays, color may confer no benefit at all. In response to my inquiry, Steve Ryan, program manager for Energy Star’s power-management program, asked consulting firm Cadmus Group to run a quick test by loading Blackle, Google and the Web site of the New York Times (which is, like Google, mostly white on-screen) on two monitors — one CRT, one LCD — and connecting a power meter to both. “We found that the color on screen mattered very little to the energy color consumption of the LCD monitor,” said David Korn, principal at Cadmus, which specializes in energy and environment, and does work for the government. The changes were so slight as to be within the margin of error for the power meter. Tweaking brightness and contrast and settings had a bigger effect. The bulkier CRT screen did see savings with Blackle of between 5% and 20%. Mr. Korn emphasized that this was a quick test, not a rigorous study.

Under about:config, change browser.display.background_color from #ffffff to whatever hex value you want.

Changing screen space to a dark color (preferably black) will ONLY make a power difference when using AMOLED screens, most commonly found on phones. So if you're using a regular LCD, TFT or, IPS display, you won't see any battery life improvements, but your browser will be swagged out

If you have Stylus extension installed, just create a new style with this exact code:

@-moz-document url-prefix(about:blank) {*{background-color:#000000;}}

_{credit goes to terdon's answer}

I know this is an old question, but in Firefox 64, the chrome folder doesn't exist anymore in the /Library/Application Support/Firefox/Profiles/RANDOM_TEXT.default location.

I did find a setting though under the about:config key browser.display.background_color, which you can set to any hex color code. This setting will probably save my eyes a couple dozen times a day.

HTH