as far as they do not catch the fire....
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It would concern me. If they are fairly close to the fireplace they will get hotter than the room temperature is. No different than if you sit to close to a campfire. At first you're comfortable, pretty soon you get hot and have to move back. Same thing. Set them up and see how warm they get. If they seem to warm, I'd move them away.
If the sides of the speakers are warm from fireplace or just sun radiation it is not optimal. My concern would be with ferrofluid widely used in tweeters but sometimes even in midrange (my speakers have 6 1/2 midrange drivers using ferrofluid as a suspension). When ferrofluid dries out (temperature speeds up the process) it has to be replaced. It can be done but it is hard to find proper people for the job.