Is there an extension that will automatically beautify and add syntax highlighting javascript files? Much like this Chrome extension:
https://chrome.google.com/webstore/detail/javascript-and-css-code-b/iiglodndmmefofehaibmaignglbpdald?hl=en
I know it's possible in the web developer tools, but I just want to load a page and have it happen automatically. I'm not looking to edit anything, I just want to be able to quickly go through a file as needed. Thanks!
Firefox 134
"Pretty print source" {} button under the source code:

Firefox 42
Firefox Developer Tools has:
"Prettify Source" button: braces
{}icon on bottom left"Auto Prettify Minified Sources" setting: turns Prettify Source on by default.
To enable it: go to the engine icon on top right of the Debugger tab, not the global settings engine.

Tested on Firefox 42.
There is now a plugin that intercepts JavaScript downloads and deminifies it at that point.
Unfortunately, the way it hooks into Firefox means that it applies to all JavaScript downloads and just not specific ones and the JavaScript files have to be served with an appropriate MIME type.
https://addons.mozilla.org/en-US/firefox/addon/247565/