To begin with, placing the "Bookmarks Toolbar Items" on the Navigation Toolbar can sometimes produce unexpected transitory conditions. Plus adding the Search Bar back to the Navigation Bar may also add to the unexpected transitory results.

I put all the bookmarks that I want on the Toolbars in one solitary folder on the Bookmark Toolbar, to save toolbar space; and then have a drop-down of bookmarks from where ever I place the "Bookmarks Toolbar Items" solitary unnamed folder.

Beyond that, the best suggestion that I have is to "restrict" the width of both the Location Bar and the Search bar thru userChrome.css code to stop those side-by-side "bars" from being able to use their "flex" properties. Doing just one may allow the other "Bar" to "flex" and use what Firefox sees as all the available space,

#urlbar-container {max-width:1050px!important}
#search-container {max-width:200px!important}

Vary the 1050 and 200 to suit your needs.

See this to learn about using userChrome.css in Firefox for customization of the UI (User Interface). https://www.userchrome.org/

thanks for comment. i use userchrome ,but it not work.

I have never had that happen as per your gif. Notta. Use the Menu Bar to slide some Customization you have dragged out, to it. Turn on the Bookmark Toolbar as a Separator and make use of that Toolbar.

Thank you I will give up. I do not want to occupy the vertical space

Using 60.2OESR on a Mac -- OS X El Capitan 10.11.6. Every time I upgrade Firefox, even a one-digit upgrade, all of my bookmarks disappear into overflow. Generally, something has changed and they reappear, but not this time. I want the three tiers of bookmarks I had before! "Hamburger" menu > Customize > Toolbars > to Bookmark Toolbars does nothing, because that was already checked. Several people mention entering code somewhere to fix it, but I am a lowly end-user and have no idea how to do that. Can somebody please help me? Thanks!