Question: Is there any method in `about:config` to enable in the URL bar JavaScript prefixed by `javascript:` to run in Firefox? 

`browser.urlbar.filter.javascript: false` doesn't seem to work.

Answer: 
### `browser.urlbar.filter.javascript: false` doesn't seem to work.

Indeed it doesn't. You can work around it using Bookmarks (at least until Mozilla block that as well):

> 
> * Set up a Bookmark with the location: `javascript:%s`
> * Set the keyword for that bookmark as: `javascript`
> 
> 
> Now you can run JavaScript from the address bar in the format:
>  `javascript alert('Hello World')` which is very similar to the old
>  `javascript:alert('Hello World')` method - it's just that the colon
>  needs to be replaced by a space.
> 
> 
> 

Source [Executing JavaScript in Firefox address bar - AutoHotkey Community](https://www.autohotkey.com/boards/viewtopic.php?t=58269)
The setting you are trying to use only controls whether javascript: URLs show up as auto completes when you type in the URL Bar. From <http://kb.mozillazine.org/Browser.urlbar.filter.javascript>

> 
> True  
> 
>  Filter out “javascript:” URLs from appearing in the Location Bar autocomplete dropdown. (Default)
> 
> 
> False  
> 
>  Allow “javascript:” URLs to appear in the dropdown.
> 
> 
> 

It seems javascript: URLs are completely disabled in newer version of firefox. 

However you should still be able to save javascript: URLs as bookmarks and activate them by clicking on them. 

Other people have noticed this too: 

<https://www.reddit.com/r/firefox/comments/9qykd0/firefox_63_disabled_javascript_in_address_bar/>