Firefox bookmarks (and browsing history) are contained in a file named places.sqlite which is in the Profile folder.

See this support article to learn where the Firefox Profile folder is located:
https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_finding-your-profile-without-opening-firefox

From there you'll need to figure out where the "backup" file is located; like where Win 8.1 would place that "backup".

Firefox uses two locations for the Firefox profile folder, so make sure to look in the correct location. Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\

Location used for the disk cache and other temporary files (Local Directory on about:profiles).

• C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\

• https://support.microsoft.com/en-us/help/17128/windows-8-file-history