Of course, a lot has been written about useful extensions for FF, but I want to talk about those that relate exclusively to improving the usability of the interface. I rarely install extensions and install only what I really need. And so, meet:

Speed ​​dial

The function is copied from the Opera. It creates a tab with nine pages of your choice, which you most often visit. Represents them in the form of thumbnails, the number may be different from nine. Links are available via ctrl + 0..9 (1..0, to be exact).
Speed ​​Dial on Firefox Add-ons

Personal menu

The main function of this extension is to provide a replacement for the main menu. PersonalMenu will hide the old menu and open it by pressing alt. Replacement is a small button located on the toolbar, where you can collect the most necessary functions from the main menu. Very interesting thing.
Personal Menu on Firefox Add-ons

Undo Closed Tab Button

Saves the history of closed tabs and displays them like the buttons "back" and "forward". The number of memorized tabs can be changed in the settings.
Undo Closed Tab Button on Firefox Add-ons
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Favonizetab

Everything is as simple as a pistol shot; when you click FaviconizeTab, only the icon remains in the context menu of the tab; header and red button disappear. You can configure the tab to collapse-expand by pressing alt / ctrl / shift + click.
FaviconizeTab on Firefox Add-ons

Thank you for your attention, enjoy your surfing!