All versions of Firefox, or just your own specific copy? Any time you notice problems in only one browser, the first step is to empty the cache. Sometimes it's enough just to reload the page, though it's not likely to work here.
Is yours an https site? Different browsers react differently to pages that have a mix of secure and non-secure content, most often in the form of secure html with ordinary non-secure images.
There exist situations where browser-specific issues have to be dealt with on the server level. But those generally involve, ahem, older versions of MSIE. Firefox mostly does what it's supposed to do.
Thanks a lot for the reply Lucy. You nailed it I think on the secure/unsecure content and how browsers react. I have taken steps to ensure all content is via https, and so far haven't had an error since, and the ie popup warning has gone away as well. Although that wouldn't really explain the intermittent nature of the problem. If I don't see the error again within the next several days we can probably assume that resolves it.