See: https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

Open Customize and use the Restore Defaults button at the bottom of the Customize panel.

Occasionally, a settings file that stores the width of the address bar and search bar becomes corrupted with nonsense sizes. If other methods of fixing this do not work, you could try this next:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show in Finder" button. This should launch a new Finder window listing various files and folders.

Leaving that window open, switch back to Firefox and Quit, either:

• "3-bar" menu button > "power" button
• (menu bar) Firefox > Quit

Pause while Firefox finishes its cleanup, then rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old.

Start Firefox back up again. Any improvement on the toolbar?

If you need to redisplay the Bookmarks Toolbar, you can use either:

• View > Toolbars
• right-click (or Ctrl+click) the "+" button or the blank area on the tab bar