The 3-bar Open Menu button is similar to the orange Firefox button that was in Firefox 4 thru 28. Has some of the menu items that are on the menu bar and has toolbar buttons that a user may not need to see all the time, and it provides access to the Customize Panel. Unfortunately that button can't be hidden easily.

Personally I haven't had a problem with that button slowing down startup, but you're on Linux while I'm on WinXP.

You can look at the Classic Theme Restorer extension to restore some functionality that was lost with the arrival of the Australis style in Firefox 29.

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

• You can check 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 in the Customize palette (3-bar Firefox menu button > Customize)

See also:

• https://support.mozilla.org/kb/how-to-make-new-firefox-look-like-old-firefox

• https://support.mozilla.org/kb/common-questions-after-updating-to-new-firefox
• https://support.mozilla.org/kb/learn-more-about-the-design-of-new-firefox

• http://forums.mozillazine.org/viewtopic.php?f=38&t=2824649