I upgraded to Firefox 41. It would crash occasionally and when it opened back up, it acted as though it was installing an update. What it was really doing was reverting to version 39. After a while, I got sick of it and uninstalled Firefox and all of its components, including all of the various folders. It was running much better and everything seemed good. Then I went to install version 42. Firefox never restarted on its own, it just closed. When I opened it up, I was at version 39 again. I'm not even sure where Firefox got a hold of the version 39 installer, since I tried deleting all of the folders. Now, I'm trying to upgrade to 42 again, and the download counter just starts over repeatedly, counting up the file size over and over. Hope somebody has some advice.