What happened when you tried to post a comment? For example, form wasn't operable, submit didn't work, site acted as though you were not logged in, site appeared to accept the comment but nothing actually was posted, etc.

For future reference, and generally speaking, when you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server. Any difference?

(You also could clear Firefox's Cache Completely using:

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now")

2. Remove the site's cookies

While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"

Then try reloading the page. Does that help?

I just noticed that you posted this same issue and received the same suggestion before: Latest upgrade will not allow comments in flickr.

In that thread you explained further:

When I click in the comment box the cursor goes to the beginning of the message "ADD COMMENT HERE....Etc" but I cannot do anything. Normally as soon as you click in the box the message goes away and then you can type. I cannot even click and drag to highlight the message.

Please note that Firefox 15 changed the behavior around "placeholder" text in form fields. That text doesn't disappear until you type at least one character. More discussion and a workaround in this thread: Installed Firefox 15 Cursor will go only to beginning of text boxes.