Check if The bookmark backup file or one of the automatic dated backups are located inside the Profiles - Where Firefox stores your bookmarks, passwords and other user data folder bookmarkbackups folder.

hello,thank you for your reply.YES bookmark backup file or one of the automatic dated backups are located inside the Profiles. BUT UNFORTUNATELY the date of these backups is only to saturday 26 the day when they disapeared so nothing BEFORE 26th is available.from 26th nothing was there anymore including 26

Did you reset Firefox or did a new profile got created otherwise?

When you reset Firefox then a new profile is created and some personal data (bookmarks, history, cookies, passwords, form data) is automatically imported and the current profile folder will be moved to the desktop to an "Old Firefox Data" folder.
Installed extensions and other customizations (toolbars, prefs) that you have made are lost and need to be redone.

It is possible to recover more data from the old profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

You can also check if there are more profile folders

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox