If I breathed pure oxygen could my lungs catch on fire because of all the tar and nicotine in there after more than 50 years of smoking cigarettes

No. For one thing there is a lot of moisture down in your lungs, for another, starting a fire takes a lot of heat, much more than human body temperature.

But there will be things in your lungs that slowly oxidize if you breathe in pure oxygen. Not so much the residue of tar left from many years of smoking, but your normal body chemistry will take a hit with all that oxygen looking for something to bond with.

I dont think it will be lethal, but you are likely to get sick. Dont try it.

No, but your skin can catch fire in 40% oxygen. No matter how long you smoked, all the crud is gone within a few months, maybe a year or so.