If you mean you don't want Firefox to close when the last tab of a window is closed;

Type about:config<Enter> in the address bar. If a warning screen comes up, press the Be Careful button. This is where Firefox finds information it needs to run. At the top of the screen is a search bar.

Type browser.tabs.closeWindowWithLastTab and set its value to False.

Edited wrong value. Sorry  :-/

i mean,if there's only one tab left in a browser window,i hope the close button(X) on the tab could be invisible,just like the version before.

If browser.tabs.closeWindowWithLastTab is the default True then you do not see the close X on the tab is only one tab is left.

You can look at the Classic Theme Restorer extension to customize Firefox.

• Classic Theme Restorer: https://addons.mozilla.org/firefox/addon/classicthemerestorer/

• You can check out the settings of the CTR extension via its Options/Preferences button on the "Firefox/Tools > Add-ons > Extensions" page.
• You can find extra toolbar buttons and more toolbar settings in Customize (3-bar Firefox menu button > Customize)