tumble-weed.png

Nobody even looked at the question after do many months... looks like this site is abandoned.

I don't think there is a way to disable the overflow functionality of the main toolbar. As you have discovered, the stretchy, resizable "Bookmarks Toolbar Items" control doesn't work very well with drag-and-drop if you put it on the main toolbar.

What if you add bookmarks a different way, using the built-in dialog instead of drag-and-drop, or this extension:

https://addons.mozilla.org/firefox/addon/bookmark-tab-here/

Is the bar any more stable?

You would have to set a suitable fixed width for the location bar container via userChrome.css to prevent all bookmarks from ending up in the overflow menu. The location bar has quite a large flex attribute that will win from other flex items you place on the Navigation Toolbar, so putting the bookmarks on this toolbar isn't a good choice like posted above.

Thanks, @jscher2000 and @cor-el for your replies. Yes, using a dialogue for adding a bookmark works (better), but it doesn't allow to put the bookmark in a certain place (of course I can move it later, but that requires additional clicks, not big deal though).

I understand the implications of overflow menu, but this just makes less sense to the very idea of this functionality. Why does it exist in the first place?

scrutari said

I understand the implications of overflow menu, but this just makes less sense to the very idea of this functionality. Why does it exist in the first place?

I don't understand your question. The overflow menu is to accommodate items that don't fit on the bar based on the sizing rules applicable to different controls at different window widths. In the default configuration, the logic of the two overflow menus calculates independently. When you put everything on one bar, it doesn't work well because it wasn't designed to work that way.

scrutari said

I understand the implications of overflow menu, but this just makes less sense to the very idea of this functionality. Why does it exist in the first place?

By default, the Navigation toolbar does not have the Bookmarks toolbar to the right of the URL bar / aka Location bar as you have it doing. That area of the Nav bar is typically used by extension toolbar buttons and the "overflow" is there to display those toolbar buttons when there are too many buttons to fit; and when the browser window is less than Maximized as with your screenshot, the "overflow" could be affected by the "float" of the Location bar AND two "flexible spaces" (one on either side of the Location bar) unless you removed those "flexible spaces" while in the Customize mode.

And as jscher2000 alluded to, you are dealing with two overflows - one for the Navigation toolbar and the 2nd for the Bookmarks toolbar - PLUS the large flex attribute for the Location bar that corel mentioned is a factor that needs to be considered - along with the two "flexible spaces" that I mention.

My recommended course of action is:

1. Get rid of the "flexible spaces" that "encase" the Location bar, if they are still there while in the Customize mode. They could provide "padding" that could "flex" when adding a Bookmark, throwing the Nav bar out of kilter temporarily.
2. Limit the width of the Location bar via userChrome.css code as corel mentioned. To remove the "float" built into the Location bar.

userChrome.css code:

#urlbar-container {max-width:750px!important}

https://www.userchrome.org/

OR - just don't move the Bookmarks toolbar to the Navigation toolbar. With Quantum that type of modification just doesn't work as well as it did in the past with Australis (Fx 29 thru 56) and versions before Firefox 29. IMO, the "overflow" in Quantum for the Navigation toolbar "screwed up" that type of modification.

I did have the "Bookmarks Toolbar items" ON the Navigation toolbar going back to Firefox 0.9 up thru Firefox 56, but ran into problems similar to what you are experiencing and just quit doing that mod after weeks of frustration trying to find "the sweep spot" that Quantum would accept. I did find "the sweep spot" with Firefox 4.0 and again with Firefox 29 to get around the problems back then, but with Quantum between the Location bar "float" and the two "overflow" toolbar features "I met my match" with that type of modification to the UI.