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QWebEngineView and Ember.js ES6

Before I begin, I know almost nothing about JavaScript. I'm just a lowly C++ developer being tasked to do something that should be easy.
In essence, I need to configure a QWebEngineView to show a webpage that is being developed for us by an outside UI vendor. Whenever I load the page in my QWebEngineView, I get a ton of errors in the JavaScript Console (http://imgur.com/vAapTZW). However, if I go to the page directly through Chrome, everything looks exactly like it should (http://imgur.com/M58umll).
I have checked that other webpages can load correctly within my view, but, for whatever reason, not the page that is being developed for us. I have also scoured the Qt documentation for references to any JavaScripts limitations and have also found none.
For reference, they are using Ember.js with JavaScript version ES6 and I am using the latest version 5.5.1 of the Qt framework, but have also tried 5.3.2. I have also tried using the now deprecated QtWebKit instead of QtWebEngine hoping it was simply a design oversight in the new widget.
Any help either solving this issue or coming up with a strategy to figure out how to debug this would be amazing!
Thanks!

Accessing native c functions from javascript

I have a javascript application which will be running on an embedded platform. It has to call some c APIs which are already implemented in the platform. This is a proprietary application and is meant to run only on selected devices.
I am running this application on webkit. There is no JVM available.
I am thinking of some options and got a few. I would like to know whether any of them are applicable or a better option. I am new to javascript so my ideas may not be right.
Something similar to JNI ( if this exists at all)
Develop a plugin for webkit which exposes some Javascript APIs to the application and then calls native c functions
Modify webkit

Your 2nd suggestion is pretty close to what you need to do. When you compile the browser (you don't need to compile webkit yourself) you can register functions and objects.
If you are using Qt, you should probably start here.
If you are using GTK, the registration is different, here is a nice example.

One approach you might want to explore is script alert handler. I know webkit gtk supports it http://webkitgtk.org/reference/webkitgtk/stable/webkitgtk-webkitwebview.html#WebKitWebView-script-alert
You can have simple javascript alert on your webview and parse parameter on other side and then call the required function. Of course, if you have complex parameters (e.g. objects) which can not be translated to strings easily, this approach will be difficult to implement.
Only problem is I don't know if this functionality is supported on your platform since you did not mention the platform, there is no way for me to check.

What's a simple way in Google Chrome to insert Javascript into sites?

I would like to be able to add custom snippets of javascript to any site that matches a regex. This is mostly because of sites that I use daily because of specialized content, but which have a poor design. I want to be able to do something like:
Visit site See that sidebar overwrites content
Whip out developer tools, find div id for sidebar
Edit a snippet of javascript which is executed on document.ready for this domain:
$('#sidebar-right').remove();
A bit of searching for user scripts tells me that I need to be writing a Chrome extension, which seems unfortunate and overkill. Is there not an easier way, or an extension which is nothing but a javascript editor that assigns snippets to domains? I'd really like to be able to edit in Chrome itself, or at least have a file that I can just leave open in MacVim all the time. An extension requires unloading/installing to update as far as I can tell.
If I just had one big javascript file, that would be fine. I'd just wrap my customizations in domain checks.
Bonus love if I can write in CoffeeScript.

The answer is to use the Tampermonkey chrome extension.
https://chrome.google.com/webstore/detail/dhdgffkkebhmkfjojejmpbldmpobfkfo

Why not dotjs http://defunkt.io/dotjs/ ? It's local, you can version it with git, you can easily take it to another computer...

Another alternative that neatly solves the problem is Custom JavaScript for websites. You just need to install the extension, which takes around 2 seconds, and then you can immediately start typing your custom JavaScript for the specified website.
The extension automatically recognizes the current website, so all you need to do is write your code and click on Save. You can also easily import jQuery or your external scripts for convenience.

Custom JavaScript for Websites 2 is an alternative to Custom JavaScript for Websites, with some bug fixes and sync scripts across devices feature.

Snippets are available directly in Chrome Devtools
https://developers.google.com/web/tools/chrome-devtools/javascript/snippets

Witchcraft is another Google Chrome extension for loading custom Javascript and CSS, and it is supposedly for more avanced users. The older dotjs project repository lists Witchcraft as its successor.

What you're looking for is Greasemonkey. But wait, Greasemonkey is for Firefox only, right? Turns out, you can install Greasemonkey user-scripts as native Chrome add-ons. Just go to userscripts.org and hit the Install button on one of them, and Chrome will automatically convert it into a native add-on. Then, write your own and install it using Chrome.
Note: This only works in Chrome 4.0.

Javascript, Firebug: How do I have multiple views or what is the right of way of doing development?

Most of my javascript work is done with Firebug and I feel annoying most of the times having to switch between the HTML mode and console mode (which again I split into output mode and input mode). When I switch to a different page to see the HTML and come back, I lose the code that I write. What is the best way to go about developing javascript applications using firebug?
Something like a mini IDE would be awesome: It just has to let me insert some code, examine the current page and then let me execute it. Any suggestions?

I use a simple text editor (vim) to write Javascript and HTML, and I check the result every now and then in Firefox. I have always two windows open: one for my text editor, and one with the current page open in Firefox. After saving a change in the Javascript, I switch to the browser and refresh the page to observe results. That was my workflow until recently.
A couple of weeks ago, I discovered the Combiner tool by Nicholas C. Zakas. To release my Javascript code, I am now using a complete build process based on Apache Ant, similar to what I was using while doing Java development previously.
The first step is to check the Javascript code with the JSLint tool by Douglas Crockford. I used to painfully copy and paste my Javascript code in the online version of the tool, once in a while; being able to run it on all my Javascript files at once with this Ant script is a huge convenience.
The second step is to combine all my Javascript files into a single file using the Combiner tool. The third step is to minify the Javascript code using YUI Compressor by Yahoo!. These last two steps allow to optimize the delivery of Javascript code to reduce page loading.
You can find an example Ant build file that you may adapt to your own needs. I am currently using this file to build my own Javascript library, bezen.org.

The other answers so far have been correct: I've never heard of anyone actually developing inside Firefox/Firebug, because it's a tool designed for debugging, not coding. To do your coding, you should use a tool designed for it (either a text editor or a full-fledged IDE).
But that being said, you might want to check out FireEclipse (Link). It will allow you to integrate Firebug with the Eclipse IDE (which itself has at least three different JS plug-ins to choose from). Alternatively I think the main (aka Web Standard Toolkit, aka WST) JS editor for Eclipse has some functionality which is similar to Firebug, but I've never used it so I don't know the details.
Hope that helps.

Write your HTML in an editor/IDE
Save your changes
Preview it in your browser
Debug using Firebug
Make your code edits in your SOURCE CODE
Repeat
Firebug is for debugging and allows you to do some "what if" fiddling while the page is live. This is not a replacement for an IDE.

Using Firebug Lite (1.2) would you use it in production sites?

I tried to use Firebug Lite (via the bookmarklet and also adding it to one of my web sites).
I seem to get the alert:
Unable to detect the following script "firebug-lite.js" ... if the
script has been renamed then please set the value of
firebug.env.liteFilename to reflect this change
Alot. Especially when I try to close the tab. This happens in Firefox, Camino and Safari.
What I'm wondering is, is this ready for use?, or do I need to copy the code, post it locally and hack it? I already checked the documentation, and it was pretty limited.
I was also looking at the mod dates and the site appears to have been relatively idle since mid-2008.

The javascript file rounds out at 77,305 bytes, so I would think you would not want to use it on a production site unless you were using a dynamic language and could output the script conditionally when you need to do debugging.
(i.e. http://www.somefakesite.com/page?debug=true)
As long as you're only including the file in the page when you are actually debugging, it probably doesn't matter where you pull the script from unless it doesn't work correctly, in which case you would have to modify and serve it yourself.