The update in question, KB982217, is labeled "Update for Microsoft Search Enhancement Pack" and marked as an Important update rather than Optional. This means the extension will be installed either automatically, if the user has Automatic Updates on, or manually when the user clicks Install, as Important updates are checked by default. What makes matters worse is that the update's knowledge base article says nothing about installing an extension or add-on.
There doesn't seem to be any security implications to this add-on's background installation, but as you can imagine Mozilla is not particularly amused by having Microsoft silently updating their browser -- and Firefox users probably won't be either. If you don't appreciate the extension cluttering up your Firefox, or simply don't use Bing's bar, it's safe to remove it just as you would with other extensions -- you can also delete the Firefox Extension folder from "C:\Program Files\Microsoft\Search Enhancement Pack\Search Helper\" to make sure it's gone from your Firefox install.