I have a few gmail accounts that I switch back and forth with. I have one regular, one for school, and one for work. From the Google homepage or from in any of the mailboxes I used to be able to click the account button and switch back and forth easily. For instance I could be in my main email click the account button and select my school and it would correctly open my school email in another tab. This no longer happens. It only continuously opens the last one I was logged in to. I saw some people trying to say that it's a Google thing and they'll only let you be logged in to one at a time but it worked before without me having to logout and in between the multiple accounts. Also this account switching seems to work fine in Chrome as well which leads me to believe it's not Google but Firefox.