Well, in that case you need to upgrade both OpenXcom and FMP.
I suggest to:
0. make a save in geoscape, with all your craft inside the bases and preferably no UFOs flying around
1. make a full backup of both application directory (c:\Program Files\OpenXcom\) and user directory (c:\Users\<yourusername>\Documents\OpenXcom\)
2. delete EVERYTHING (yes everything!) from the application directory (c:\Program Files\OpenXcom\)
3. download the newest nightly build of OpenXcom and extract it into that directory
4. copy the original UFO Defense files into the directory c:\Program Files\OpenXcom\UFO\
5. run openxcom... to see if everything still works... also it will create some necessary folders automatically
6. download the newest FMP and extract it into directory c:\Users\<yourusername>\Documents\OpenXcom\mods\
7. run openxcom... enable FMP... try loading your save... and see if everything still works
8. if yes, enjoy... if not... luckily you have a backup