Focus Time works with Firefox, sort of

As I write this, I am sitting here with Focus Time turned on and one of my often visited -2 rated sites open in a new tab. However, going to the same site in IE8, the page is blocked immediately - Focus Time is clearly functioning perfectly here. And, strangely enough, if I go back to firefox after doing this, Focus Time appears to start working in firefox - including blocking other -2 rated sites that have not been visited in IE - albeit after some time. There is a considerable wait until Focus Time blocking steps in - until the page has finished loading at least, and then some - and even then, the block can be worked around relatively easily by simply revisiting the original URL! None of these problems are present in IE, however - as previously stated, IE works flawlessly (for once).

So, my question is - what gives? I would really like to know what is going on here, because under IE I have found Focus Time to work really effectively, but compared to firefox it is nowhere near as functional for me to use... or so I thought. Has anyone else had this problem?