Windows XP SP3 is still supported by Firefox 48. You can use the internal updater: Update Firefox to the latest release.

As for whether it's a good idea... There are security fixes in newer versions, and there may also be higher demands for CPU, memory, or graphics capability for some features, while new efficiencies may have been added for other features. So it's a little difficult to predict in advance.

Hello,

Marc7 said

I am using an older version of Firefox on my XP Machine. I am thinking of updating it since Firefox is getting to the point where I don't know how much longer older versions of the browser will work even on an XP device. I have seen there is a way to update to the latest version is you're using and older machine, but I am curious as to whether or not you even still support XP at all. Is it worth trying to update? My machine runs on Service Pack 3. I have a Windows 10 laptop that I planned to update to the latest version, but my bigger concern is whether or not updating my XP machine's firefox is even possible and/or worth the attempt.

I think till the end of the year you will be able to use your xp, but jscher2000 is correct when he said ....There are security fixes in newer versions, and there may also be higher demands for CPU, memory, or graphics capability for some features, while new efficiencies may have been added for other features. So it's a little difficult to predict in advance.

see also : https://support.mozilla.org/questions/1108395

thank you

Hmm. Almost sounds like it's not worth worrying about then if XP will not be supported past the end of the year. Hell, as my machine runs on a Pentium 4 processor, I'm not even sure I can update anyway. I suppose I'm better off staying with what I have or at least keeping a copy of the older version around. I thought Firefox was continuing to support XP though since not everyone can upgrade or purchase new machines?