Seems like versions available at different URLs keeps changing.
For version 126.96.36.199 I found a copy at http://www.majorgeeks.com/mg/getmirror/ ... fox,2.html
If you cancel automatic install, and instead do a right-click and Save Link As on the flashing banner "Click Here if it does not", then you can save a copy of .xpi file to your local hard drive. Then you will have your own copy if you need to remove and re-install again in Firefox if something goes wrong with future updated versions and you want to go back to 188.8.131.52 later and it is no longer available online at the usual sites.
Similarly, you can right click and Save As copies of 4.4.1 or other older versions, available at other FF add-ons sites, if you want to have a local copy, to allow options for installing, removing, and changing which version you want to use.