This is well known but does not show manually modified preferences rather ANY modified preferences as the name implies. Even out of the box on a fresh install this results in pages of "modified preferences" that didn't even have a user initiated trigger.
This shows also shows preferences modified in indirect ways such as about:preferences, devtools, etc. Helpful to filter a list of potentially manually toggled prefs, but not a 1:1 filter.
On about:config to the right of the search bar at the top is a checkbox "Show only modified preferences".