You can found the name of default menus in menus.cfg file in folder:
\K-Meleon\defaults\settings
And in some macro files in macros folder.
In your case you are searching Tab context menu is in menus.cfg file:
# ---------- Context Menus -----------------------------------------------------
# ----- Tabs
%ifplugin tabs
TabButtonPopup{
!New
!CloseTab
!NavTab
-
:T&abs
}
%endif
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