Hmm, I'm not sure exactly what's happening. I'll take a guess:

When Firefox starts laying out a page, it often knows the exact amount of space that an image will take up, but sometimes it doesn't and adjusts the position of other things once it does know that. If you are clicking on something at the moment Firefox makes that adjustment, then as you say, the click will miss the target. Other than being very patient until everything loads, or stopping the page load by pressing the Esc key (or the "x" button at the right end of the address bar), I don't know of a solution for this.