Sorry to hear about this problem. Are you able to add new bookmarks right now? If not, the relevant files might be locked. Try doing a regular shutdown/restart of Windows to clear file locks. Next, do you recall doing a Refresh/Reset/Tune-up as part of your update? This would move your entire old profile folder (your Firefox settings/data folder) to your Windows desktop inside a folder named Old Firefox Data. Do you see anything like that? If neither of those is relevant, try the first section of the following article about checking for other profiles: [[Recover lost or missing Bookmarks]] The rest of the article also could be helpful. To update us on your progress, please return to this thread on the forum (if you hover your user picture in the upper right, a panel drops down where you can access a list of your questions).

See also: *https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update