FF is fully XUL-browser that means both pages and GUI (user interface) are rendered by the engine.
K-Meleon doesn't use XUL for GUI - it utilizes native Windows GUI libs. That's why KM is significantly lighter then FireFox in terms of system footprint.
The reverse side of it is that KM cannot use FF extensions as they are - they need manual adaptation to work inside KM.
But KM has its own macrolanguage that allows to replace a number of FF extensions with native KM macros with the same functionality. Also it allows to adapt some other complex FF addons some time.
There are a number of extensions already created and adapted for KM.
Look at
KM Extensions Central for short annotations and download. First of all read the
'Privacy and Security' chapter