It doesn't work.
I have my yahoo email account as my "home page", and I noticed every morning, or whenever I first open my browser, that the usual ads are 'there'. It's only when I refresh the page, that adbock takes care of the ads, and they disappear.
Maybe some kind of adjustment can be made in future updates...
When you launch your browser it loads your start page before it loads extensions. Then all the extensions you've installed start gathering the information they need to work. In AdBlock's case, it's fetching the filters from the filter lists you're subscribed to. Those lists contain a LOT of rules, and it takes a small but noticeable amount of time to fetch them all. If your start page finishes loading before AdBlock can pull in all the rules, unblocked ads can appear on that page. When you refresh the page, the ads are gone because AdBlock has had time to fetch and process the filters.
To get around this, set your start page to the New Tab page, or instruct your browser to launch with the New Tab page open first, followed by the pages you want to open automatically. Or, as you discovered, just reload the page. :)
AdBlock Community Manager
Perhaps another fairly easy trick you could use, is to see whether you can Pin a Bookmark somewhere on the Start Page or New Tab Page, that should provide near one-click access to your Yahoo Mail account while still allowing AdBlock a chance to Update :)