Firefox ''can'' sync your bookmarks between different installation of Firefox if you set that up, but it's not automatic. Unless you set up Sync, all your data is stored only locally on your computer. When you did the reinstall, did you use the Windows 10 Refresh feature? Normally that gives you a choice to leave your data in place, create a temporary backup, or flush it. It sounds as though it was not left in place. Can you find this folder: C:\Windows.old If so, in order to see whether your old Firefox data is in there: (1) Set Windows to show hidden files and folders. This article has the steps: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files (2) Click through to this folder: C:\Windows.old\Users\''your-user-name''\AppData\'''Roaming'''\Mozilla\Firefox\Profiles The Profiles folder might contain only one semi-randomly-named folder, or multiple folders. If there's only one, click into that one, if there are multiple folders, you can check each one. Inside the semi-randomly-named profile folder, look for a folder named '''bookmarkbackups''' and click inside. Here you would expect to find files with names similar to: bookmarks-2017-09-29_3456_''gibberish''==.jsonlz4 The data coded into the file name shows when the file was captured, and the number that follows is the number of bookmarks and bookmark folders recorded in the backup. This file is compressed, and not human-readable. However, it can be restored to your current Firefox in place of the bookmarks you see there now. To do that: (A) Copy the backup file to your Desktop or another convenient location. For example: right-click the backup file > Copy Go to your Desktop, right-click a blank area > Paste (B) Inside Firefox, use the Restore menu, Choose File command, to point Firefox to your desktop and grab the old backup file. This article has more details on the Restore feature: [[Restore bookmarks from backup or move them to another computer]]. Hope that helps.