Type in your location bar – about:config and Enter
you will see a message – Click on I’ll be careful, I promise (if your open first time)
Then in the filter bar type – browser.tabs.closeButtons
Double click on browser.tabs.closeButtons
By default value is -1
Firefox display a close button on every tab also depend how much tab you have open.

When you open firefox on full screen-

Maximum open tab with close button – 5 (800 x 600 screen resolution)
Maximum open tab with close button – 7 (1024 x 768 screen resolution)
Maximum open tab with close button – 9 (1440 x 900 screen resolution)

If you open more tab according to your screen resolution, Firefox display close button setting automatically changes and now Firefox displays a close button only on the currently open tab.

Set default value – 0
Firefox display a close button only on the currently open tab.

Set default value – 2
Firefox don’t display close button on any tab.

Set default value – 3
Firefox don’t display close button on any tab but it show end of the tab bar in right side.

Note: Don’t change about:config setting without full confidence & knowledge

But if you don’t want to wait, use this simple trick.

Type in your location bar – about:config
You will see a message – Click on I’ll be careful, I promise
Then in the filter bar type – security.dialog_enable_delay
Double click on security.dialog_enable_delay  and set value 0 (By default value is 2000)

Restart your Firefox, now you can install add-on without wasting time.