Like VotePlugin (for which I posted a fix), FiveStarVotePlugin currently introduces invalid XHTML 1.0 strict, which is what Trac normally produces:
Validation Output: 1 Error
- Error Line 63, Column 147: document type does not allow element "ul" here; missing one of "object", "ins", "del", "map", "button" start-tag.
…a total of 5"><ul class="star-rating"><li class="current-rating" style="width
The mentioned element is not allowed to appear in the context in which you've placed it; the other mentioned elements are the only ones that are both allowed there and can contain the element mentioned. This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.
Embed the <ul/> part into <object/>.
This works for me using Firefox 2 and IE 6.
Note, though, that I'm by no means an expert on (x)html
Patch diff fix attached.