Firefox XP support will shift to ESR 52, drop in mainline past 51

Chris Cooper, Release Engineering Manager at Mozilla, states:

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We've decided to move Windows XP to the Firefox 52 ESR branch, so that Firefox 51 will be the last mainline version to support the platform.

We still don't have an official EOL date for XP support, but moving the platform to the ESR branch means that we will *not* need to worry about new feature support for XP which is becoming increasingly difficult.

Edit: FF 52 is currently scheduled to be released on March 7, 2016.