The creation of a new save will wipe out an old one.. unless that is something different with firefox.. I do most of my testing in chrome but my secondary testing is done in firefox.. The only thing I know about firefox and the saves that do differ is that the file name / folder if it is different from previous then it will not load old saves...
I will try running the next version through firefox for creating and saving.. to see if I can replicate any issues.. and then work out how to fix them.. if I cant replicate them, it can be kinda hard to work out how to fix them..
Thanks for the comments though as its always handy to see if anyone is having a problem as if more than one person mentions the same thing, I can start considering its more of game issue that a local issue..