Users running Firefox browser can't get past the Captcha

[mikeGoblin]

A lot of users tell me they can't register with my embedded Nabble  forum, and after some investigation it seems that if they are using Firefox the captcha simply always tells them it failed. If they try Safari (for example) it succeeds.

I have lost a lot of users because of this problem, if anybody has a workaround to fix this I would be very grateful to hear it :)

Many TIA!