When you go to view the source code of a website in Firefox, the code is syntax highlighted like in the screenshot below. Some people seem to love syntax coloring, while others are annoyed by it. And while there are no options in the Firefox Preference menu for disabling it, the more advanced about:config will allow you to disable syntax highlighting in the source code.

1. In the location bar, enter about:config and press enter.
2. In the new page that appears, there should be text field labeled Filter. In this, type in view_source.syntax_highlight and then press enter.

3. Double click anywhere on the entry's line. It should become bolded and read False under the Value column.

Now you can try going to View>Page Source on any page as you normally would. You should see the lack of syntax coloring, such as below with the about:config page itself. No restarting Firefox required.

If you pefer to not have syntax highlighting anywhere, you can disable it in Emacs and Vim as well.