I can't comment on a Blogger blog. It has CAPTCHA (word verification). It keeps telling me I've done it wrong when I'm certain I've gotten the CAPTCHA right. It's my favorite blog and I've been a contributor for a while, but recently the blog added CAPTCHA because of spam.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.
I have had this problem for a couple of months now as well. I use Firefox 18.0.1 on a Macbook Pro 10.8.2.
I have no problems at all commenting on Blogger blogs using Safari or Chrome. Only Firefox. The thing is I like Firefox best and would like to the ability to comment on blogs using it - I could do so in the past with no problems.