New
#1
As long as there are old versions of FF available there shouldn't be an issue for Xp and Vista users browser wise.
Software companies are one by one giving up on Windows XP support for their products, and now it appears that it’s Mozilla’s turn to switch the focus to newer versions of Windows.
Firefox 53 will be the first version of the browser which will no longer support Windows XP and Windows Vista, so users who haven’t yet upgraded to Windows 7 or newer will have to either stick with Firefox 52 or move to a different browser.
Installation of the new version will be blocked on Windows XP when Firefox 53 launches in March 2017, but the good news is that Firefox 52 will be moved to the ESR (extended support release) branch, so it’ll continue getting patches until mid-2018.
Read more: Firefox 53 Will Drop Support for Windows XP and Windows Vista
As long as there are old versions of FF available there shouldn't be an issue for Xp and Vista users browser wise.
All versions and builds of Firefox are available here Directory Listing: /pub/firefox/
I've not had any word from Mozilla that this will be changed in the future. The only problem with using older versions will obviously be security issues.