Ironically, an amateur developer coding up a plug-in for Firefox will be much less likely to have to worry about the issue than a large company. Because most smaller developers use the Mozilla Foundation's Add-ons download site, they are more likely to be secure.

Google, for example, not only uses an unsecured connection to check for and download updates, but also suppresses any notification that an update is being installed, Soghoian said. The company has already created a patch and will automatically be updating Google Toolbar users soon, a representative said.

"We were notified of a potential vulnerability in some updates for Firefox extensions," the representative said in a statement sent to SecurityFocus. "A fix was developed for the Google extensions and users will be automatically updated with the patch shortly. We have received no reports that this vulnerability was exploited."

Soghoian, who spent last summer interning at Google, hopes that the search giant will also reconsider its decision to update users without notification. Firefox users now need to take a critical look at the third-party add-ons installed on their browser, and having as much information as possible helps, Soghoian said.

"As a matter of general policy, vendors really should not have their software silently install updates without asking the user's permission -- it is asking for trouble," he stated in his advisory.

The Mozilla development team is currently considering ways that they could prevent insecure updates in the next version of the browser, Firefox 3.0, the group said on its blog.