In Firefox, one thing you can’t customize is the main menu. And you may find it’s a waste of space to have a whole bar for just a couple of frequent tasks like bookmarking, history or tweaking a preference here and there.
Personal Menu is a Firefox extension developed by Merci Chao that comes to solve this. By default it adds two powerful toolbar buttons that mimic the Bookmarks and History menus and perform quick actions like opening the History sidebar or bookmarking a web page when middle-clicked. A third button is waht really gives Personal Menu its name. You can set this button to display a custom menu you modify by adding as many menu items and subitems as you wish, even mixing them to create the menu that better suites your needs.
For example, since I tend to try new extensions frequently I need the Add-ons item on hand as well as the Options… (from the Tools menu), Downloads and the awesome FireFTP, followed by all the other menu items I access less frequently.
Once all your preferred buttons are in place, right click on a toolbar and uncheck the Menus Toolbar to hide it and save some space and get a less busy window at the same time.
If you realize you hid it and forgot to keep a way to customize Personal Menus and access some option, Personal Menus says you can press Ctrl + Shift + Alt + S to restore it. However I didn’t get my main menu back with this hotkey, so make sure to at least add the Add-ons menu item somewhere in your personal menu.