For HeaderControl to work with K-Meleon 74.0 and K-Meleon 75.0 Beta 1. download HeaderControl
https://addons.cdn.mozilla.net/user-media/addons/11327/headercontrol-0.1.7-fx.xpi and rename it to {dee093e0-1eb4-11de-a5df-0002a5d5c51b}.xpi
Then download UAheadercontrol
http://kmext.sourceforge.net/km74/UAheadercontrol-km74.7z for K-Mleleon 74 and extract macros to the k-Meleon Root Folder.
In K-Meleonn 74.0 and K-Meleon 75.0 Beta 1, the macros from UAheadercontrol, allows us to set a Custom User Agebt String.
List with Links to Firefox Addons that work for 74/75
http://kmeleonbrowser.org/forum/read.php?9,131158