Answer

There are several reasons why this might occur in some browsers but not others:

Caching issues

The browser is caching the page; have your visitors refresh the page after signing in to force the page to update.

Your pages are encoded with UTF-8.

Some browsers are more likely to cache the page when you use UTF-8 character encoding in conjunction with the sign-in Javascript.

Try changing the character encoding in mt-config.cgi¹:

PublishCharset iso-8859-1

This presumes that your templates are using this code by default:

<meta http-equiv="Content-Type" content="text/html; charset=<$MTPublishCharset$>" />

Upload mt-config.cgi back to your server in ASCII mode, then rebuild your site and check the sign in then.

You are using multiple domains.

If your Movable Type installation is on one domain, but your weblog is on another, this can cause problems with Internet Explorer, due to that browser's designation of certain cookies as "leashed".

"Leashed" cookies are new to Internet Explorer 6. They're cookies that Internet Explorer prevents from being used later, by a third-party site. So when the TypeKey commenter cookie is set (for the domain where your Movable Type installation is located), IE won't allow that cookie to be used on your weblog, because it thinks it is a third-party cookie.

In this case, anyone who wants to comment on your site will have to make changes to their privacy settings in Internet Explorer, by following these steps (substituting your actual installation domain name for domainname.com):

1. Go to Tools > Internet Options, then select the Privacy tab.
2. In the Settings area of the tab, click Sites.
3. In the Address of website field, type domainname.com and click Allow.
4. Click OK at the bottom of that dialogue box.
5. Click OK on the Privacy tab.

This change should now permit the sign in cookie to be set correctly.

¹ In versions prior to 3.2, the configuration file is named mt.cfg.