It seems that the latest few version of Firefox are in some way caching the captcha image. As a result, te captcha you enter into the field is always incorrect. I've seen it a few times before on other sites, and by using B2J Contact I've experienced it on my site as well.
Searching for the solution I came across a few "hacks", but I'm not sure if I can use any of them in the B2J component (and in what file) without breaking it. Perhaps someone can suggest me the right way to do this:
1. on Stackoverflow a developer has placed

header("Cache-Control: no-cache");

header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");

in some file, and set the captcha img src to

Since the captcha problem was still occuring, another developer suggested that he should fix the Last Modified header to send the current time.

2. There is a simple advice on Digital Point which suggest that "when you reload captcha using javascript (right?) append a random dummy number as a query to the captcha image src, e.g