I haven't had occasion to try commenting on either a Blogger or Blogspot blog, so I checked a couple past threads on this. Didn't see anything confirmed other than accepting third party cookies.

You listed several add-ons that might have their own separate restrictions on cookies. Checked these?

• BetterPrivacy 1.48.2 "Super-Cookie Safeguard"
• Ghostery 2.2.1
• Permit Cookies 0.6.2

To see whether a plain vanilla installation of Firefox works, you could, as a diagnostic, create a new (blank) profile: Managing profiles. You don't have to use it going forward, but if you can comment in a default configuration, then it's probably an add-on causing the problem.

You can start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Oh, thank you, thank you, a thousand thank yous. If I were paying your allowance, I'd increase it!!

Apparently, Ghostery has a block on Google FriendConnect and I needed to disable that one block in order to post on the embedded comment form.

Thanks, again.

And I cant write comments when later a CAPTCHA code is required. In most cases and for most pages the properly typed CAPTCHA code fails with Firefox. In that case I always switch over to IE which handles CAPTCHA codes fine.