See:

• http://kb.mozillazine.org/Lost_bookmarks
• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

• https://support.mozilla.org/kb/Lost+Bookmarks
• https://support.mozilla.org/kb/restore-bookmarks-from-backup-or-move-them

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

Your important data and their files Bookmarks, Downloads and Browsing History

   places.sqlite 

This file contains all your Firefox bookmarks and lists of files you have downloaded and of websites you have previously visited. For more information see Create bookmarks to save your favorite webpages. Passwords

Your passwords are stored in two different files, both of which are required:

   key3.db - This file stores your key database for your passwords. To transfer saved passwords, you must copy this file along with the following file.
   logins.json - Saved passwords. 

For more information see Password manager - Remember, delete and change saved passwords in Firefox. Site-specific preferences

   permissions.sqlite 

This file stores many of your Firefox permissions that are decided on a per-site basis. For example, it stores which sites are allowed to, or blocked from setting cookies, installing extensions, showing images, displaying popups, etc. Search engines

   search.json 

This file stores the search engine data. For more information see Search bar - add, change and manage search engines on Firefox. Personal dictionary

   persdict.dat 

This file stores any custom words you have added to Firefox's dictionary. If you have never added a custom word to the Firefox dictionary, then you will not have this file. For more information see How do I use the Firefox spell checker?. Autocomplete history

   formhistory.sqlite 

This file remembers what you have searched for in the Firefox search bar and what information you have entered into forms on websites. For more information see Control whether Firefox automatically fills in forms with your information. Cookies

   cookies.sqlite 

Cookies are used by websites for a variety of reasons, some of which include keeping you logged in to a website you have previously logged in to, remembering any preferences a website has allowed you to set, authenticating you as a person who has visited a specific website before, etc. Security certificate settings

   cert8.db 

This file stores all your security certificate settings and any SSL certificates you have imported into Firefox. File types and download actions

   mimeTypes.rdf 

This file stores your preferences with respect to what Firefox does when it comes across known file types, such as whether you want to save a file or open it with a specific application. For more information see Change what Firefox does when you click on or download a file.

cor-el said

See:

• http://kb.mozillazine.org/Lost_bookmarks
• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

• https://support.mozilla.org/kb/Lost+Bookmarks
• https://support.mozilla.org/kb/restore-bookmarks-from-backup-or-move-them

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

I give up! I will recreate my bookmarks, they have disappeared into the ether!

Hi, the easiest thing to try is - open your bookmarks > Show all Bookmarks > Import And Backup (top of the page) > Restore, and hopefully your bookmarks will still be there.

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

FEBE (Firefox Environment Backup Extension) {web link} FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backup of files that you choose.

OPIE {web link} Import/Export extension preferences