You should not lose your settings when upgrading! However, you may see a message that Firefox is starting slowly and you can speed it up by restoring some default setting. ''Do not click that button unless there is a serious problem!'' That button performs a Refresh, which definitely undoes your toolbar customizations. Usually the Refresh doesn't erase bookmarks or passwords, but some users have had to recover them from the "Old Firefox Data" folder the Refresh creates on your desktop. You'll probably have more peace of mind if you make a backup of your Firefox settings folder. This article has tips on some options for doing that: [[Back up and restore information in Firefox profiles]].