C:\Users\youroldusername\

C:\Windows.old\Users\youroldusername\

Was this an "in-place" type of upgrade where your user folder C:\Users\''youroldusername''\ was untouched, or did you end up with a new user folder? In the case of a new user folder, can you find a folder named C:\Windows.old\Users\''youroldusername''\ If so, your Firefox data probably is in there -- and here is how I suggest accessing it: First, it would be helpful to set Windows to show hidden files and folders. This article has the steps: http://windows.microsoft.com/en-us/windows/show-hidden-files Then click into: C:\Windows.old\Users\''username''\AppData\Roaming\Mozilla\Firefox\Profiles Note: there is a cache folder under AppData\Local, but all the important stuff is under AppData\Roaming. Here you may find one or more semi-randomly named folders. The one with the most recently updated contents probably is the one you want to restore. This post has my suggested procedure for resurrecting that data in your new installation of Firefox. Please note the use of ''copy'' instead of ''move'' throughout the process. This allows you to try again if it fails somewhere along the way. https://support.mozilla.org/questions/1024189#answer-638319 Any luck? Note, if you really ONLY want bookmarks, copy the two latest backups from the old bookmarkbackups folder to a convenient location such as the desktop, then see this article (you'll use "Choose file" on the menu to get to those backups): [[Restore bookmarks from backup or move them to another computer]]. Note that this process will replace any new bookmarks you might have saved since installing Firefox again. If you need to preserve those, use the export feature first: [[Export Firefox bookmarks to an HTML file to back up or transfer bookmarks]].

Note that the name of a JSON bookmarks backup file includes a total item count (folders and separators included) and an hash value to prevent saving the same backup more than once. *bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4.