This is basically a reprise from bug 28387, we might want to reconsider this. I opened this bug, because of the discussion in bug 371179. IE gives a confirm dialog when you want to add a bookmarklet to your bookmarks, so maybe we want to do something similar as IE?
I think we should come up with a new UI for "user-script buttons" to replace the concept of bookmarklets. Mixing these buttons with bookmarks is suboptimal and constraining in many ways.
Before I file a new bug, I wanted to check if this would be an appropriate bug to hijack. We want to add UI that would require more explicit action by a user before they can install or execute a bookmarklet. One option is to add a checkbox per bookmark in the Bookmarks Manager that would be required to be checked before the bookmark is permitted to run script. Another option is to require a bookmarklet be "installed" before it can run, and to make the bookmarklet installation appear similar or identical to the add-on installation flow. Let me know if you want a new bug for this, or if we can do the work here.
Given the recent social-engineering aspects of this (and the general suckyness of user warnings, in general), I think we should just disable/ignore attempting to bookmark such URIs (are there other cases? Should we whitelist to just http/https/ftp?) I'd probably be ok with allowing existing bookmarklets to work if it's not a hassle. One thing I'm not sure of is how successful an attack would be that asks users to modify an existing bookmark to use a js URL. [EG "nice hack to look anyone's facebook page! Just bookmark facebook, blah blah edit it to js:// blah blah, now visit FB and click the special bookmark!"] I suspect that would still work well enough. :( Given that bookmarklets are already kind of an edge-case use (although semi-popular), killing them entirely is something we should consider.
Comment 1 is asking for a different bug to be filed and fixed. Whatever we do for user scripting, bootkmarklets are out there and they won't go away quickly. /be
For data: URLs, fixing bug 656823 would be better.