How to change firefox Find/Search text background color

Type about:config to the address bar.

Create two entries: (Right click | New | String)

ui.textHighlightBackground, value #0000FF
ui.textSelectBackgroundAttention, value #FF0000

Other properties to change:
    ui.textHighlightBackground
    ui.textHighlightForeground
    ui.textSelectBackground
    ui.textSelectForeground