One of them has been obviously noticed the last few days and it appears to be a graphics bug. I have found no problems with the CSS or HTML code as I have not modified the code since the last update.
As far as it concerns the usage, I can only conclude it is either a browser problem or a bug in Tumblr. I have encountered a few of these already when I first created it, but it reverted back to its original layout over time.
If I may suggest for any FF users, please download Firebug and see if you can identify any code errors. In the meantime, I will do my best to solve this problem.