Yahoo email is slow as fuck and likes to freeze the browser. Scrolling down in a folder with emails takes years when it contains 1000 or more emails. Trying to checkmark all emails by one click doesnt work and will only affect the visible emails, but not those older emails who need to get reloaded when scrolling down within a folder.
And last but not least Yahoo´s behaviour to spy on everything and analyze email content (The American disease) makes this site a huge scam.