Comment 168 for bug 255263

The truth is that in this case it wouldn't help you much even if plugins _and_ addons were in a separate process. Here the addon is doing valid stuff invalidly :) . In other words, once the user gives the addon permission to run, the addon has access to Chrome (conceivably to _enhance_ the UI). If it does something that isn't supported it may break the UI. To guard against every possible outcome of poor coding in addons is impossible. I think the resolution for this bug is accurate being that:

1) Mozilla provides a "Safe Mode", allowing you to use Firefox without any addons.
2) You can always disable/uninstall addons.
3) The addon developer is the one at fault, and should be the one to fix this.

In direct answer to your question: "lock the address bar" is very generic, afaik there are numerous ways this effect can be achieved, none of which can be guarded against. Say, for example, an addon developer decides to replace all addresses typed in the address bar that aren't valid urls with an about: page explaining to the user how to correctly navigate the internet. Say the about: page isn't properly registered with firefox, now every time the user types in "google", which default handling sends you to a search page with that search term, in this case the user will get an alert that firefox doesn't recognize the protocol. Who's fault is that? Now just remember, this is only one possibility (although highly improbable) out of dozens of other mistakes extension other can make.

On another note: this bug has been marked INVALID, and this is generally considered bugspam, so I'll discontinue discussing this any further here. If you'd like to propose ideas, a better place would be the newsgroups http://www.mozilla.org/community/developer-forums.html .