I thought one of my Windows file cleaners/junk deleters was causing this, so I tried saving individual website bookmarks without any use of the file cleaners (which do not run automatically), and still no success. When I save a bookmark via the menubar, I visually check that it was saved in the bookmarks drop-down menu. Upon closing Firefox, then making sure Firefox terminated by looking at Windows Task Manager as it sometimes hangs for a few seconds, and then starting Firefox once again, all bookmarks saved in the last session are gone. I can and have backed-up the bookmarks and saved them to a private folder as I have routinely done for so many years, but I have not tried to restore or import html saved bookmarks to see what happens in that instance because I know the problem I'm dealing with right now with bookmarks is not normal. I have experienced this with Firefox default theme, Curacao v4.5 which I have had no previous problems with, and persona themes. I also recently re-installed Windows Xp Home on this machine via PC vendor supplied recovery, re-updated to SP2 since SP3 drastically slows down my PC, and had previous problems with plugins such as Microsoft's drm and dotNET installing without permissions from me, but perhaps installing themselves as requirements for such software applications as I have used in the past and been reading about. That may be (or may have been) a separate problem, but it bears mentioning because it is something the end user does not expect to see or occur, meaning, it behaves like malware.