Check for that extension's button in the box of available buttons on the Customize screen. Assuming it is there, you can drag it back to your preferred spot on the main toolbar.

Customize Firefox controls, buttons and toolbars

Does that work?

Yes I worked buy do you know how to add addons which are not in tool bar .I have few addons in my addons menu which are not in tool bar.

Sometimes when the toolbar is quite full, a button will slip onto an overflow menu. An icon will appear next to the menu button with a character similar to » on it.

Not all extensions have a toolbar button. Some extensions integrate through the address bar -- check the ••• "Page Actions" menu -- and some integrate through a right-click context menu. Some have no visible user interface and need to be managed through the Add-ons page.

If you think an extension is supposed to have a toolbar button but it isn't being generated, you might try contacting its author to see whether this is a known problem.

i guess those add-ons have no tool bar button option .Thank you very much.