Very few users actually have XP corporate licenses, the vast majority of XP users haven't received an update in 4 years.

I'm not sure what you should do for browsers. We aren't supporting XP any longer. You could custom build Firefox I suppose, but the user-base isn't large enough for us to continue supporting it

DavePoirier said

What alternate browsers can I use?

If Microsoft is still supporting/updating your Windows XP, does that mean it is still supporting/updating Internet Explorer 8 on XP? The key components of IE are actually part of the OS. You may want to check with them on that.

And if you still keep Firefox after EOL, check the security page from time to time to determine whether there are any mitigations you can adopt, or simply to know when you have no choice but to pull the plug.

https://www.mozilla.org/security/advisories/

DavePoirier said

Since XP support continues for corporate licences, why are you ending support early? I have several PCs on XP Professional licenses that Microsoft is still updating. Only the Home licenses are ending support.What alternate browsers can I use?

Regular WinXP users have not really got much update support since Microsoft made it EOL back in April 2014. The so called hack for updates meant for ATM's and POS terminals and such is not really proper updates for regular users.

Mozilla has been generous in still supporting Windows XP and Vista for as long as they have already as they could have ended a year or two ago.

No real good alternatives in web browsers that still supports the EOL WinXP and Vista OS's as Chromium, Chrome, and Opera ended support back in April 2016.

By the time Fx 52.9.x ESR is EOL on Sept 5 it will have been almost 2 1/2 years longer.

Originally the legacy 52.8.x ESR was going to be the last however the current ESR is based on 60.0 and not 59.0 so it was delayed for one more update in overlap. So there is the 52.9.0 ESR release out today (June 26) (along with 61.0 and 61.0 ESR).

One of the many complications in supporting WinXP.

from the tracking Bug#1130266 for Reading is comment #16

Jim Mathies [:jimm] on Sept 29, 2016 Supporting XP is actually become quite a problem for our releng teams in that data center testing hardware doesn't support XP anymore. So for example we currently use AWS for most of our testing, but have XP tests running on old hardware we have to maintain ourselves in a data center. It's issues like this that push us to move XP out to an extended support release where we can decommission most of our automated testing associated with it.

note: Please read https://bugzilla.mozilla.org/page.cgi?id=etiquette.html before considering commenting in any of the related Bug reports like the above as "Why?" and such comments will be spam and likely hidden.

I do not really count any third-party Firefox builds as possible options as even those third-party builders who relied on the legacy Fx 52 ESR channel will be at a end also (in Sept) and back porting security and stability fixes as time goes on may be hard in keeping it secure and stable.