Depends on the organ, and how damaged it may be from the effects of CF. Don't forget that the skin is an organ, so the safest (and possibly) best way for us to be organ donors is to donate some skin for someone who may need a skin graft.
There are multiple ways to indicate how you want to be a donor. The most commonly known way is to affix the organ donor sticker to your driver's license. You may also indicate in your will or advance directive how you'd like to go about donating.
The screening is VERY strict for organ donors, so it's difficult (though not impossible) for CFers to donate. If you feel very strongly about gifting a part of your body, there are ways to do it. You may also want to consider donating your body to research at the time of your passing. If you live in the U.S., visit organdonor.gov which has more information about it.
<a target=_blank class=ftalternatingbarlinklarge href="http://educatedguesser.blogspot.com/search?q=gift+of+life+organ+">The Gift of Life</a>