I have user who lost all of her bookmarks last February . Today, she noticed that her bookmarks have reverted back to the bookmarks she lost. Could any updates impact this?
Occasionally, users who've had their systems for a long time will have 64-bit Firefox in the 32-bit Program Files (x86) folder, where it replaced 32-bit Firefox, but then an updater gets confused and creates a second installation in the 64-bit Program Files folder. Firefox will use different profiles (data folders) for each installation. It's worth checking whether there is a dual installation, and choosing one or the other to make the default browser.
To check possible alternate profiles, see the steps in: https://support.mozilla.org/kb/recover-lost-or-missing-bookmarks