Attempt #3: make that dialog less intrusive; do not require acknowledgment. Result: users get trained to overlook it; users who accidentally enable the mode will never figure out what happened to their browser.
Attempt #4: a whitelist of allowed scripts. Problem: users will disagree about what should make it into the whitelist.
Attempt #5: the JSBlock extension. This may have merit. So, if you want this feature, download it, or write it if it doesn't exist yet. If the API does not allow writing it, bicker Mozilla.
The webpage you are viewing may
not work correctly because the following
options differ from their default values:
1. Enable automatic loading of images.
These features of Firefox are essential
for most webpages to run properly. If the
webpage you are trying to access is behaving
strangely or appears to be working incorrectly,
<a href="">click here</a> to load the page
with the default browser configuration.
Also: how is that not a dialog? It presents a message to the user, and waits for a reply.