First, if you are using Firefox 3.6 you need to update to Firefox 16.0.2 IMMEDIATELY. Firefox 3.6 is not only full of dozens of known security holes, it has many more known bugs such as crashes, freezes, website incompatibilities, etc. Please update Update Firefox to the latest release then check if your problem is gone. If not, please come back and give more details and screenshots.

@ Tylerdowner: I don't want to update. I don't like the new versions of Firefox. I like that one, that I'm using right now.

Got some new info: the bookmarks only dissappear when I start the browser by clicking on the desktop shorcut. If I start Firefox from the quick launch, then no problemo, bookmarks are where they're supposed to be.

And please don't reply with "update to version 100.25.2666...etc." Update is last resort.

Staying with Firefox 3.6 is both EXTREMELY Insecure (you are vulnerable to almost every hole on https://www.mozilla.org/security/known-vulnerabilities/firefox.html since Firefox 12), and you are using a version that is significantly slower, doesn't support modern web technology, and has known bugs (crashes, freezes, etc.). If it's just the UI you don't like, you can quickly and easily change the UI to look exactly like 3.6.

So, until you update you are knowingly using an old version of software with dozens of known bugs.

Bookmarks that are stored in the unsorted bookmarks folder won't show in a drop down menu list (Bookmarks Menu button or Bookmarks menu).
That folder is only visible directly in the Library and bookmarks sidebar.

You will have to move them to the Bookmarks Menu folder or Bookmarks Toolbar folder or use the Bookmarks sidebar (Ctrl+B) instead.

• http://kb.mozillazine.org/Sorting_and_rearranging_bookmarks_-_Firefox

Partially solved it. Good enough for me.

Thank you Tylerdowner and cor-el.