First I will say you don't know what you have unless you have a medical diagnosis.
That said, even if you may have AFib, it should be checked out.
The main symptom problems that can be dangerous is blood clots and high heart rate, well over 100 bpm. A simple aspirin a day will help reduce the risk of a clot (but even this should be discussed with a doctor before taking aspirin long term - more that a week or two). There are no over-the-counter aids to slow down the HR. If you have a high HR it will require a prescription by a medical doctor. I take Metoprolol to slow down down my AFib driven high HR. I also take a prescription anticoagulant. Both are generic drugs and while more costly than aspirin they are not extremely expensive. Both are affordable if prescribed... if you see a doctor remind him/her if you prefer generic versions of drugs. In my experience most doctors do indicate generic substitution is approved if recommended by the pharmacist. More than you asked for, but maybe of some help.
When you said how serious should you be about a fib i would say pretty serious. The main question is was this diagnosed by a heart doc? Are you currenly on any medications for it? There are many of us on this site that have or have had a fib and yes it is serious. Heart rates can go into the 300's which is not a good thing and like Jerry said the main risk is clots. A fib is easily kept under control by a good doc for most people but its not something you want to take a chance with. Things to avoid if you are sure you have a fib and it was diagnosed by a doc is no caffeine, no colored sodas, no energy drinks, no red wine, cut back on the sugar, drink lots and lots of water because dehydration can set them off, get alot of sleep at nite. These are just the basics. Does a fib kill? Yep if it goes untreated but with treatment there may be options out there for you to get rid of it permantly......if you haven;t seen a doc see one....if someone else is giving you advice don;t listen.....self diagnosis is a bad bad thing......
In A-Fib, electrical signals from other parts of the heart disrupt your heart's normal rhythm and cause the atria to beat or quiver on their own sometimes as rapidly as 600 times a minute. Afib is very common and some people have it and do not have a clue in the world they have it. It is some what serious but surely not as big of deal as the ventricle arrhythmia's. A-Fib is a condition you can manage with treatment and proper lifestyle changes.
My primary caregiver is the one that found out I had A-Fib( she ran an EKG ). I now have Cardiologist also. They say that my A-Fib is not curable now. I'm on warfarin,blood pressure medicine and simvastatin. And I'm only 58 yrs. old and still working.