I haven't heard of any malware altering Firefox's bookmarks in situ in the places.sqlite database file, but I suppose it's possible.

More likely would be a bad add-on, or perhaps contamination from a bookmark synchronization service.

In addition to investigating those possibilities, she should clear history for the unwanted site. In the Library dialog that appears when choosing "Show All History" you can put that domain in the search box to find and remove those items. Then they will no longer come up on the autocomplete list below the address bar.

As for cleaning up existing bookmarks, it's difficult to think of a convenient way to globally change them. There is a button in the Library dialog to switch between History and Bookmarks search to find the corrupted ones and edit them individually.

Problems with bookmarks and history not working properly can be caused by a corrupted places.sqlite database file.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

You can try to check and repair the places database with this extension:

• https://addons.mozilla.org/firefox/addon/places-maintenance/