HI, it has been that way for a bunch of versions, maybe since 57. If you put your mouse up to it it will change to it's colours and if click it you can take the tour. It more or less comes alive if your doing a clean install and will show the box/info for the tour.

You can hide this onboarding icon by modifying this pref on the about:config page.

• browser.onboarding.enabled = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config