I've encountered a few issues where this add-on has been butting in outside of Flash videos and/or games, causing some odd problems.
In Gmail, for instance, the upload files button is somehow expanded by this add-on, preventing access to some of the text option buttons. PDF files seem to reload (which is especially problematic with larger files) so that FlashResizer can add a resize bar to the side, which, for some reason, lets me resize the entire PDF viewing window (scrollbar included). YouTube videos must be manually resized when attempting to upscale the video size.
Overall, while effective in what it does, has some unwanted effects as well.