I'm using a mac with OSX 10.6.5, and for a while now, and by that I mean since I bought survival mode, minecraft in my browser has always not worked right. Well, it's worked, but it interferes with the tabs in the same window. If I'm playing minecraft in one tab then switch to the next with minecraft still running, whether it be in pause or inventory, the game window will show up in the middle of that tab. I don't know how to fix it, but it's really annoying and if I want to play minecraft, I need to do so in a different window.
So I decided to use the downloaded version which isn't really that much better of an improvement, but it is superior so I can play offline. I recently got the new launcer (well, 'new' as in whatever the most recent one is, I didn't update it for a while) and it just went to a black screen as soon as I logged in. Even if I wait for a few minutes, it's still a black screen.
So now I have a broken downloaded minecraft, and a bad browser minecraft.
Note: I have bootcamp with windows 7 64 bit. These problems have never occurred there.