Attempt #1: show that dialog as soon as a page tries to load some JavaScript. Result (I'm guessing): dialog shows up on 99.9% of all web pages one visits (even for the nerdiest of nerds)
Attempt #2: silently download the JavaScript to figure out whether it is 'benign' or 'evil'. Result: users complain that they pay for downloading stuff they do not want.
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.