[Plugin: Contact Form 7] Captcha field on Contact 7 won’t accept text (Firefox only)

Contact 7 is great and has been working fine. But now the captcha input fields on my forms have stopped working in Firefox. I can input text on all form fields, but captcha field won’t allow me to click and enter text.

This is a problem in Firefox for Mac (I’m using Leopard) and XP. I tested it on Safari for Mac and it works properly; also tested on IE 7 for XP and it works properly.

I upgraded to latest version of Contact Form 7 (1.7.6), but that didn’t solve the problem. Tried clearing browser cache, but that didn’t work. It seems like an issue with Contact Form 7, since it’s a problem on both platforms.

Has anyone else encountered this? Is there a solution? Thanks!