Extensions in the Mozilla world do not follow any security model, and Firefox, for instance, trust extension code completely. They aren't isolated or sandboxed, and can even modify one another. Any Mozilla application supporting extensions suffers from this issue, and the code is completely platform independent. It can lead to fully compromised systems.
When looking into this problem, researchers found several issues in popular Firefox extensions Sage 1.4.3, InfoRSS 220.127.116.11, and Yoono 6.1.1 (including all previous versions). A flaw in Sage, for instance, allows a malicious RSS feed to change your NoScript settings, adding sites to NoScript's whitelist. Flaws in IngoRSS and Yoono can even lead to fully compromised systems.
Maybe some attention to the security problems in the extension framework pushes Mozilla to think about some way to change this framework to solve or mitigate the issue. In fact, Mozilla Labs' Jetpack project indeed proposes to use a sandbox security model, but this model is as of yet unimplemented in the test releases.