lascetic said

When I use yahoo to view my mail on android firefox browser, I cannot log out. I do not use yahoo app. I just use the browser to view my mail. Then when I try to sign out, I cannot.

You mean, you select the sign-out link or button on Yahoo Mail and either nothing happens or the page reloads/redirects with you still logged in? I'm not sure it matters, but is Yahoo displaying a mobile site or a desktop site?

I have to set firefox to delete everything when I quit and still it will work only if I actually tap on quit, but not if I just close the browser.

That's normal. If you tap whatever button displays images of running apps, you would normally see that backing out of Firefox, or closing websites so there's only a home tab, doesn't quit it. That's why a Quit button is added to the Firefox menu when you set up the feature to clear history at shutdown.

Yes, I tap on the sigh out link, the page loads as it should but I'm still logged in.

If I don't hit the quit "button" I remain logged in even if I close all apps (including ff) that run at the background.

I just found out a way to log out from yahoo. If I sign out from the link below my name in yahoo mail, it does not work. However, if I sign out or exit (depending on the browser) from the link at yahoo home page then it works! It is a bit weird but at least it is safer.