The following steps should do the trick:

1. type "about:config" into your address bar (without quotes) and hit enter
2. read & accept the warning
3. copy/paste: "ui.textSelectBackgroundAttention" (without quotes) into the search bar
4. change the radio selection from "Boolean" to "String" and then click the "Plus sign" button on the right hand side
5. enter your desired color (as a hex code including the '#')
6. click the check mark button to save
7. restart Firefox

Thank you. Is there a separate, additional transparency setting for the search highlight color? Black text is now obscured by the color I selected.

I don't believe there is any way to adjust the transparency/opacity of highlighted colors.

Your only real alternative would be to change the color of the highlighted text. The only issue is that, unlike with "ui.textSelectBackgroundAttention", search-highlighted text uses the same color setting as manually highlighted text ("ui.textSelectForeground") instead of having it's own separate setting. You can change "ui.textSelectForeground" using the same method as above but it will affect any text that's highlighted, whether from searching or manually with the cursor.

I haven't tried this but, I thought that you might be interested:

What is the HEX code for Transparent color? https://stackoverflow.com/questions/1801580/what-is-the-hex-code-for-transparent-color

Note that Firefox may change colors or swap foreground and background colors to "EnsureSufficientContrast".

See also the source code line #3789 and further down:

• https://dxr.mozilla.org/mozilla-release/source/layout/generic/nsTextFrame.cpp