You may have a problem with a corrupt file, for details see https://support.mozilla.com/kb/Toolbar+keeps+resetting

Thank you for your tip, but unfortunately it didn't work. I even tried deactivating all Addons, but even then I can't move the Button.

The Bookmarks button (with the drop-marker arrow) that automatically appears on the Bookmarks Toolbar when the Menu bar is hidden can be a little tricky to move. You didn't mention where you are trying to move it, but if you keep it on the Bookmarks Toolbar and want it to be to the right of those other two buttons (also on the BTB?), try moving those other two buttons to the left of the Bookmarks button.

That works for me on 4.0 with WinXP.

You too, thank you for your help. Sorry, i forgot to mention that. I wanted to move the button (the one with the dropdown menu) up into the navigation bar just left of the address field.

I tried to move the button within the addon-toolbar and that works so far pretty fine. But as I said, that's not where I want the button to be. And there is another interesting detail I just realized that I overlooked it: When i try to apply a button position in the navigation bar the button jumps automatically back down into the addon-toolbar. As soon as i reopen the customization menu the button is back in the same place I put him the first time (left to the adress bar). So in the customization menu the button keeps being at the right place, but as soon as I leave the menu the button jumps "out of order" :)

Sorry, that specific Bookmarks button (with the drop-marker down arrow or bookmarks word) is tied to the Bookmarks Toolbar when it is shown, and only appears when the Menu bar is hidden and the Firefox button is showing. It is not a "regular" toolbar button.

There is the original Bookmarks button, in the Customize Palette, which toggles the Bookmarks Sidebar view. It will stay where you put it on the Toolbars.

Did you try to move the bookmarks menu button out of the Add-on bar?

You should to move that button to a visible toolbar, provided that the Menu bar and Bookmarks toolbar are hidden. I place that button at the left end of the tab bar. There is very tricky CSS used to show a specific bookmarks menu button.

Thank you the-edmeister this "solves" not the problem but the question. :) Not so well done by Mozilla then, that it "allows" you to moved that button without really allowing it. Or maybe it's just a hidden April Fool by Mozilla ("oh yes, let's move the button up there...April, April, that doesn't work!") ;)

@cor-el: Yes, that's what I tried Unfortunately I don't really get your answer, maybe it's my limited English.