Are you a new Firefox user?

Firefox has been doing that with Tabs on Top selected, (for me on WinXP), since Firefox 4.0 was released. Using Customize and moving that Bookmarks button only works when the Bookmarks Toolbar is deselected from appearing; with the BTB showing, that button has special properties to force it to appear on the BTB.

That action hasn't bothered me since I always have used the Bookmarks Sidebar to view and use my bookmarks.

I thought I was fairly confident with my Firefox usage having used it not long after it came out, however I could not figure out why the Bookmark icon would not stay on the left side.

I did go and uncheck the Bookmark Toolbar and it finally showed up on the Left side like I wanted it too. However I lost all my little bookmarks along the top where I could access them with just a click of the button, instead of going through my various folders.

I guess there must have been an issue with 17.0 and the previous set up I had because I wasn't having this issue prior to updating yesterday.

Interesting, I just spent a few minutes playing around, checking and unchecking various options and if I decide to have the Menu Bar showing, I lose the Bookmark tab all together.

I don't like the new look with 17.0, but can't please everyone and I'm sure I'll learn to deal with it. At least I have my bookmarks on the Left, making it easier to go through and find things in there. Either that or I seriously need to cut down the number of sites I bookmark.

Yes, the Bookmarks Menu button is meant as an alternative for the Bookmarks menu on the menu bar, so the BMB is hidden if you have the Menu bar visible.

As you've noticed that if the Bookmarks Toolbar is visible then the Bookmarks Menu button will always be on the Bookmarks Toolbar.
You can drag the BMB button with the drop marker to the left end of the Bookmarks Toolbar to have it displayed there instead of at the right side.
Firefox doesn't care as long as it is on the Bookmarks Toolbar.