Your system details list shows:

• browser.display.use_system_colors: True

Does it help if you toggle this pref to false and let web pages set their colors?

I see that your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts.

The user.js file is only present if you or other software has created this file and normally it wouldn't be there. You can check its content with a plain text editor (right-click: Open with) if you didn't create this file yourself.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can cleanup the prefs to have a fresh start and delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

• http://kb.mozillazine.org/Preferences_not_saved

Thank you for taking the time to answer my query, I followed your suggestions - I toggled the browser setting and I removed the prefs.js + user.js files but the problem remains.

Do you see the visited forum pages in the history?

Note that it is possible the the forum uses a threaded view at the left and opens the pages in a frame at the right and in such a case you may not see the visited state.

Thank you, yes the pages are in History.

Unfortunately it is not restricted to one forum:

www.boards.ie forums.watchuseek.com/f20/ foot.ie f30.bimmerpost.com/forums/forumdisplay.php?f=412 forum.tz-uk.com

Some of my favourites, All affected under 32, all fine under 31

What changed?