It's possible that the add-on is installed but disabled. You can check here:

orange Firefox button or Tools menu > Add-ons > Extensions category

The disabled extensions are grayed out and listed below the enabled extensions.

If you find it there, the most likely reason is that it includes a list of compatible Firefox versions and it hasn't yet been updated for Firefox 6. If it worked on Firefox 5, there is a good chance it will work on Firefox 6, but no guarantee.

You can force Firefox to ignore add-on version restrictions by (what else) installing an add-on. Then you can test whether the add-ons important to you actually work in Firefox 5.

Add-on Compatibility Reporter :: Add-ons for Firefox

After you install this add-on and restart Firefox, there will still be a message that "--- is incompatible with Firefox 6.0" but the add-on will be enabled and a Disable button will appear for that entry.

Ok ... problem solved (sort of) by Norton ... listed here Solution

However ... even though I've got the toolbar and ID safe the toolbar is not listed under Firefox Add-ons ... curious ... I'll wait to upgrade to Norton 2012 and/or next firefox release as I wouldn't be surprised to have it break again.