There is no extra fire danger in a 100% Oxygen environment as long as the partial pressure of Oxygen is identical to typical seal level pressure or slightly less. The extra fire danger (as in what happened with the Apollo 1 fire) comes from a 100% oxygen atmosphere at standard sea level pressure. That is a fire just begging to happen with almost any material.
BTW, the Apollo spacecraft used a 100% Oxygen atmosphere because it was less mass to haul up to the Moon and back (thus more Moon rocks to bring back and more stuff to bring to the Moon in the first place). The Apollo astronauts seemed to have done just fine with that for a week or so in space at a time, and in fact the Skylab environment was also 100% Oxygen (with CO2 scrubbers in both cases to pull that gas out of the mixture as it was produced).
The reason the Space Shuttle went to a more normal 80/20 mixture of Nitrogen to Oxygen ratio had more to do with the electronics they were using than anything about the astronauts themselves. Since electronics are designed to operate here on the Earth, an assumption is made that other kinds of atmosphere environments won't be used by anybody using those components. Yes, milspec equipment can be made to overcome that problem, but sometimes things like test equipment and a whole bunch of stuff being used inside of the Shuttle simply can't be made economically with that strict standard.
Interestingly enough, the space suits used for EVAs still stuck with the 100% Oxygen environment. One of the reasons for that is because of the lower pressure made it easier to bend joints... something sort of important if you want a practical space suit. The downside is that it takes longer for astronauts to get in and out of the airlocks.
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