Personally I close most of my apps through the Recent Apps button anyway, but if you want to have an Exit button in your menu there are two ways of getting it:

1. Set your browser to clear data on exit. (Settings - Privacy - Clear private data on exit)
2. Get the QuitNow addon.

Thank you for replying.

1st.this Addon says, not compatible for your platform. 2nd.like other known browsers there must 'Exit/Close' in this browser also. 3rd.there is a drawback in using addon. After every installation, I need to search and add addon manually.

In my opinion, closing an app via the recent apps list is faster than going to the menu and searching for the close button. Besides, a lot of apps on Android don't have an exit button either. I've no idea why it poses a problem for you in Firefox. Besides, I just checked - there is no exit button in Chrome either.

What do you mean a drawback? How often do you clear data and reinstall an app?