Did this start to happen recently, or have you always had this problem? My instinct tells me that it might be a third-party add-on that is causing this, but it's hard to tell which one it might be.

One thing you could try is to Reset Firefox, which will reset settings to default while keeping your personal data such as bookmarks and passwords: https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems

Hope this helps!

Thanks for your recommend but it cannot solve the problem. Anyway, it is never show that button until now I see it for a while when my mbp has slow down (i opened so many application) and it disappear very soon after that.

Please, any solution.