You will need to install a user script manager extension to install this script.
Version 2.3.0 is the first version with GM4 compatibility, with script menu commands missing compared with Tampermonkey or Violentmonkey.
This script allows you to specify sets of words that you want highlighted in different colors on websites every time you visit. Thank you to everyone for your comments on the beta version here: New Beta Version Available for Testing.
You still need to specify the sites using the script's settings on the Firefox Add-ons page or Tampermonkey dashboard. Hopefully in a future version that can be built into the script itself.
Based on erosman's Text Highlighter script from userscripts.org. An earlier variation on Text Highlighter is available as Text Highlighter - Dynamic.
To get an idea of how it works, please see the following screen shots:
(1) The script displays a small "H" button in the upper right corner of the page. You can click that button to display the control bar.
(2) You can display a list of the keyword/style sets from the bar. Some samples are included for reference.
(3) Use the Seek button for a set to enable the buttons on the control bar.
(4) and (5) Use the Edit button to open a panel to modify the keywords or associated styles. Note that any color or background-color rules in "Custom" will override the controls above it. Please note: the script does not clean your input!
Version 2.2 adds ability to edit keywords on the set list (double-click to show controls). The Revert button is only to the immediately previous keyword.
(6) Use the Add New Set button to create a new set.
(7) Use the Options drop-down on the control bar to turn off the "H" button or to not highlight in <pre> and <code> tags.
(8) Firefox users can conveniently access settings through the right-click context menu.
Note: The "monkey menu" has a option to flush out all of the new keyword/style sets so you can start testing as though it was a fresh install. (Not in Greasemonkey 4.)