As I said (and gave you a link for), the ESR version will be fully functional and supported until June 2018 eight months from now. But yes, the normal version of Firefox hasn't worked and has refused to install in XP since April 19 this year. If you don't like that I hear they will give you a 100% refund of what you paid for it.
With the POSready 2009
registry change, XP has received security updates every month since normal XP support ended in April 2014, and will continue to receive updates and MSE definitions until April 9, 2019. Keep in mind these updates do not include the parts of XP that aren't in POSready 2009 such as the multimedia bits. As even an updated OS isn't much use if there is no current browser for it, hopefully someone will continue producing one after June 2018.
Vista of course ended support on April 11 this year, so XP is actually more up-to-date than Vista.