Hello,

It is possible that the bookmarks have moved to another folder in the same directory. You can check for a new created profile folder when you type about:profiles into the search address bar and find a profile that isn't named "default" or "default-release."

If not, you can find the old Firefox data folder in C:\Users<username>\AppData\Roaming\Mozilla\Firefox\Profiles. If you find it, open the bookmarkbackups folder and copy the files, and place it in your current in-use profile.

If you had sync enabled before the update, logging in again could fix the issue.

If Firefox starts up after an update without your bookmarks and other settings, check whether your data is available in a different profile:

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it. The About Profiles page will open. This page should list at least one profile and could list many.

The profile that Firefox is currently using will show:
This is the profile in use and it cannot be deleted.

Please do not delete anything here; don't use any Remove buttons.

Do you have another profile named default or default-longnumber listed on this page? If so, you can check whether it is the one you were using recently by clicking its Launch profile in new browser button.

If it isn't what you want, simply close that new window.

If it IS what you want, click the Set as default profile button for that profile on the about:profiles page. Firefox will use it automatically at the next startup.

For more information see Recover user data missing after Firefox update or reinstall.

Note: If your bookmarks are not available in another Firefox profile, see Recover lost or missing Bookmarks for other solutions.