Firefox nightly updates pretty much never hurt your data and very rarely are broken significantly enough that you can't use them for your daily work. I haven't

Firefox nightly updates pretty much never hurt your data and very rarely are broken significantly enough that you can't use them for your daily work. I haven't had to revert more than two or three times in the last 5 years because of nightly bustage. If you're really that concerned, just create a second profile and shortcut to Firefox with the -P "profilename" option and you're all set.