Hi narutowindy, there is no built-in setting for this.

Firefox supports applying custom style rules to the user interface using a file named userChrome.css in a special location. Your request actually has come up before on the subreddits related to Firefox. The complexity is that the "title" of a new tab isn't completely consistent between versions of Firefox or different languages. But for Nightly you could try:

#main-window:not([title^="Nightly"]):not([title^="New Tab"]) #PersonalToolbar
{
  visibility: collapse;
}

More info and variations:

• https://www.reddit.com/r/firefox/comments/79lrur/autohide_the_bookmarks_bar_after_new_tab_just/dp34zij/
• https://www.reddit.com/r/firefox/comments/6uuqil/show_bookmarks_toolbar_on_new_tab_page/dlvobzi/
• http://kb.mozillazine.org/UserChrome.css
• Steps to create userChrome.css on Windows (Linux may be similar?): https://support.mozilla.org/questions/1164362#answer-978078

I've edited the above reply to remove a sparse trailing comma.

#main-window:not([title^="Nightly"]):not([title^="New Tab"]) #PersonalToolbar,

where should i create userChrome.css file ?? on linux ??

It goes here:

[profile folder]/chrome/userChrome.css

In addition to the above links, see: Profiles - Where Firefox stores your bookmarks, passwords and other user data

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

• create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• use a plain text editor to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• paste the code in the userChrome.css file in the editor window
• make sure that the userChrome.css file starts with the default @namespace line