Microsoft ended support for Windows XP, but, Windows XP users still exist a lot.
However, cloud services that do not support Windows XP future might come more.
This extension is automatically overwrite the version that you specify the version of the Windows part of UserAgent.
It does not affect the other parts even upgraded the Firefox because rewriting the part only Windows version.
You can get details about UserAgent from Mozilla Developer Network
"Gecko user agent string reference"
"The Future of Developing Firefox Add-ons | Mozilla Add-ons Blog"
"Add-ons/developer/communication - MozillaWiki - Upcoming Changes in Add-on Development"https://wiki.mozilla.org/Add-ons/developer/communication#Upcoming_Changes_in_Add-on_Development