Use system colors: Check this option preference to use the colors defined in your operating system settings instead of the colors specified above.

Click on the color samples to select colors. Text and Background: Here you can change the default text and background color to be used on web pages that haven't specified that information. In the window that opens, set Override the colors specified by the page with my selections above to Always.Some websites have custom font colors, to disable them: Select the color you want by clicking one of the colored rectangles. Clicking on any of the colored rectangles will show you the possible colors you can choose from.Under Fonts and Colors, click the Colors… button.Text Encoding for Legacy Content: The text encoding selected here will be used to display pages that don't specify which encoding to use.This is useful to prevent sites from using overly small fonts that are barely readable. You can also set the minimum web page font size. You can also change the size for Monospace fonts. Specify the font to use for Serif, Sans-serif and Monospace fonts.Then specify the font size you want for proportional text. Select whether proportional text should be serif (like 'Times New Roman') or sans-serif (like 'Arial').For a language/script not in the list, choose Other Writing Systems. For instance, to set default fonts for the West European languages/scripts (Latin), choose Latin.From the Fonts for drop-down list in the Fonts dialog, choose a language group/script.In the Fonts dialog that opens, uncheck Allow pages to choose their own fonts, instead of your selections above.Under Fonts and Colors, click the Advanced… button.

Some websites have custom fonts to disable them:

Any changes you've made will automatically be saved.

Under Fonts and Colors, use the drop-down menus to select the font and font size of your choice.

In the General panel, go down to Language and Appearance.

Click the menu button and select Settings. Click the menu button and select Options Preferences. In the Menu bar at the top of the screen, click Firefox and select Preferences.

Change firefox colors