about:config in the address bar.
ui.textHighlightBackground and change its color value — you can use the #RRGGBB format or generic names such as "green".
Note this also changes the highlight color of the matched text and that Firefox applies a default transparency value to the marks, so that a pure
#FF0000 red becomes fainter. There does not seem to be a way to disable that transparency filter.
References and attribution: Bug report 1 and bug report 2.