I finally compiled K-Meleon. I figure I should compile the plugins next. I went ahead and tried.

When I try to compile bmp_menu, I get this:
Compiling...
bmp_menu.cpp
d:\dev\kmeleon\bmp_menu\bmp_menu.cpp(711) : error C2065: 'MIIM_FTYPE' : undeclared identifier

external compiled fine.

When I try to compile fullscreen, I get this:
Compiling...
fullscreen.cpp
Linking...
Creating library Debug/fullscreen.lib and object Debug/fullscreen.exp
fullscreen.obj : error LNK2001: unresolved external symbol "char * __cdecl SkipWhiteSpace(char *)" (?SkipWhiteSpace@@YAPADPAD@Z)
fullscreen.obj : error LNK2001: unresolved external symbol __imp__SHAppBarMessage@8
\projects\mozilla\mozilla\dist\WIN32_o.OBJ\Embed\kplugins\fullscreen.dll : fatal error LNK1120: 2 unresolved externals
Error executing link.exe.

Trying to compile history generated 51 errors and 4 warnings...

For ie_favorites:
Linking...
nafxcwd.lib(dllmodul.obj) : error LNK2005: _DllMain@12 already defined in LIBCMTD.lib(dllmain.obj)
nafxcwd.lib(afxmem.obj) : error LNK2005: "void * __cdecl operator new(unsigned int)" (??2@YAPAXI@Z) already defined in LIBCMTD.lib(new.obj)
nafxcwd.lib(afxmem.obj) : error LNK2005: "void __cdecl operator delete(void *)" (??3@YAXPAX@Z) already defined in LIBCMTD.lib(dbgdel.obj)
nafxcwd.lib(dllmodul.obj) : warning LNK4006: _DllMain@12 already defined in LIBCMTD.lib(dllmain.obj); second definition ignored
nafxcwd.lib(afxmem.obj) : warning LNK4006: "void * __cdecl operator new(unsigned int)" (??2@YAPAXI@Z) already defined in LIBCMTD.lib(new.obj); second definition ignored
nafxcwd.lib(afxmem.obj) : warning LNK4006: "void __cdecl operator delete(void *)" (??3@YAXPAX@Z) already defined in LIBCMTD.lib(dbgdel.obj); second definition ignored
Creating library Debug/favorites.lib and object Debug/favorites.exp
\projects\mozilla\mozilla\dist\WIN32_o.OBJ\Embed\kplugins\favorites.dll : fatal error LNK1169: one or more multiply defined symbols found
Error executing link.exe.

Layers:
d:\dev\kmeleon\layers\layers.cpp(1143) : error C2065: 'RBBS_USECHEVRON' : undeclared identifier

loader compiled fine.

macros:
d:\dev\kmeleon\macros\macros.cpp(1448) : error C2374: 'i' : redefinition; multiple initialization
d:\dev\kmeleon\macros\macros.cpp(1434) : see declaration of 'i'
d:\dev\kmeleon\macros\macros.cpp(1892) : error C2065: 'BIF_NEWDIALOGSTYLE' : undeclared identifier

ns_bookmarks:
ns_bookmarks.cpp
d:\dev\kmeleon\ns_bookmarks\ns_bookmarks.cpp(475) : error C2065: 'RBBS_USECHEVRON' : undeclared identifier
ns_bookmarks_edit.cpp
c:\program files\microsoft visual studio\vc98\include\map(58) : warning C4002: too many actual parameters for macro '_Tr'
c:\program files\microsoft visual studio\vc98\include\map(62) : warning C4002: too many actual parameters for macro '_Tr'
c:\program files\microsoft visual studio\vc98\include\map(208) : warning C4002: too many actual parameters for macro '_Tr'
c:\program files\microsoft visual studio\vc98\include\map(212) : warning C4002: too many actual parameters for macro '_Tr'
ns_bookmarks_utils.cpp
c:\program files\microsoft visual studio\vc98\include\map(58) : warning C4002: too many actual parameters for macro '_Tr'
c:\program files\microsoft visual studio\vc98\include\map(62) : warning C4002: too many actual parameters for macro '_Tr'
c:\program files\microsoft visual studio\vc98\include\map(208) : warning C4002: too many actual parameters for macro '_Tr'
c:\program files\microsoft visual studio\vc98\include\map(212) : warning C4002: too many actual parameters for macro '_Tr'
d:\dev\kmeleon\ns_bookmarks\ns_bookmarks_utils.cpp(968) : error C2065: 'DWORD_PTR' : undeclared identifier
d:\dev\kmeleon\ns_bookmarks\ns_bookmarks_utils.cpp(968) : error C2146: syntax error : missing ';' before identifier 'childMenu'
d:\dev\kmeleon\ns_bookmarks\ns_bookmarks_utils.cpp(1087) : error C2668: 'insert' : ambiguous call to overloaded function
Utils.cpp

op_hotlist:
Linking...
Creating library Debug/op_hotlist.lib and object Debug/op_hotlist.exp
op_plugin.obj : error LNK2001: unresolved external symbol "void __cdecl quicksort(char *,unsigned int,unsigned int,int (__cdecl*)(char const *,char const *,unsigned int),unsigned int)" (?quicksort@@YAXPADIIP6AHPBD1I@ZI@Z)
op_utils.obj : error LNK2001: unresolved external symbol "void __cdecl quicksort(char *,unsigned int,unsigned int,int (__cdecl*)(char const *,char const *,unsigned int),unsigned int)" (?quicksort@@YAXPADIIP6AHPBD1I@ZI@Z)
op_utils.obj : error LNK2001: unresolved external symbol "int __stdcall EditProc(struct HWND__ *,unsigned int,unsigned int,long)" (?EditProc@@YGHPAUHWND__@@IIJ@Z)
Debug/op_hotlist.dll : fatal error LNK1120: 2 unresolved externals
Error executing link.exe.

rebar_menu compiled fine.

toolbars compiled fine.

Quote

op_plugin.obj : error LNK2001: unresolved external symbol "void __cdecl quicksort(char *,unsigned int,unsigned int,int (__cdecl*)(char const *,char const *,unsigned int),unsigned int)" (?quicksort@@YAXPADIIP6AHPBD1I@ZI@Z)

quicksort is defined in qsort.cpp

Quote

fullscreen.obj : error LNK2001: unresolved external symbol "char * __cdecl SkipWhiteSpace(char *)" (?SkipWhiteSpace@@YAPADPAD@Z)

SkipWhiteSpace is defined in Util.cpp

Sorry, those are the only ones I understand. For undeclared constants, you can try searching MSDN to see if there are alternative names supported in VC6, otherwise it's usually fine to just delete them.

c:\program files\microsoft visual studio\vc98\include\map(58) : warning C4002: too many actual parameters for macro '_Tr'
[...]

This one is tricky, it seems the stl in VC6 is using something called _Tr, but I've redefined it to something else ...