Change website colors in Firefox
Solution 1:
Select Tools > Options > Content
Then the "Colours" button.
Select white for the "Text" colour and black for the "Background", not forgetting to clear the "Allow pages to choose their own colours, instead of my selections above" option.
You might need to make sure that the "Use system colours" option is cleared as well.
Solution 2:
Stylish is your friend. It is the best (and the only?) tool for this.
You can find a lot of styles for Stylish on the following site: http://userstyles.org/