If that had been the case, all the History should have been deleted and not just six days worth.
Tosh, can you type 'about:profiles' in the address bar of your Firefox browser in a new tab and check if it lists more than one profile? If that is the case, click on 'Launch in a new browser' underneath the profile not in use and see if you can find your missing bookmarks in that window. To avoid repeating this, delete the second profile after you have retrieved your missing data.
Another option is that you have been using a 'private' window for the duration of the period in question. I am not sure about this one though as I was under the impression that bookmarks created in a private window would be saved in the normal way.