Firebug is, finally, going to be part of Firefox developer tools http://blog.getfirebug.com/2014/11/10/firebug-3-next-generation-of-firebug/ What they should have worked on since day one but nobody cares, they use Chrome 
What is what is not crystal clear but marriage is the future.
“Web Developer” is Web Developter Toolbar? https://addons.mozilla.org/en-US/firefox/addon/web-developer/ Just one more tool. If Mozilla did not have their weird priorities they would pick up why so many like it and get “inspired”. Can be much more useful than Firebug and Chrome. Dev. seem ok but has also dived in to Chrome land, notice when it was last updated.
I use 2 profiles and Dev. edition. Could stuff most in to one but is also a matter of spreading out the load. If you do web dev. you should also have a 100% clean one for testing purposes. Just having something like LastPass running can inject stuff and then who knows. You can also have 10, what ever make sense.
I only disable Mozilla reporting/telemetry because I highly expect it to be misused for tracking so they can optimize and user enhance their ad business. ANY middle man will do that. Is inevitable. That is all with regards to “security/paranoia”. I will expect what ever from them. Remember they are newbies in selling ads. Is why they start slowly. At some point they wont make clumsy announcement but just do it. Might also get greedy later on, advertising has its own way to create need for more.
If you read Ghacks you will also know Mozilla allow very questionable extensions in their repositories. He have shown in detail how they operate and reported it. Nothing happened. Plus I have seen at least one very popluar extension with tracking stuff like Google Analytics build in, track every click. Is in direct violation of their submission rules and nobody cares. I will expect quite a few do this. Hey ho, is open source and we can remove code our self. Is so great.
So before stuffing user.js with weird entires may be you should evaluate where “threat” or annoyances come from and take it from there. Less is often better right? In any case it might be better you check what CLEAN extensions there are on add-on site before thinking it is all up to you.
But yeah you probably have to manually disable “outgoing connections” your self. Try search on Mozillazine forum. Especially right now this might be work in progress. As far as I know this http://kb.mozillazine.org/About:config_entries plus the links is the official info.
If you think it stinks then check Chrome extensions
You also have to click and read slowly there. Well check Chrome itself, wireshark away. Pick your poison.
They still operate via Bugzilla, open source and all that, so you could start following progress for what you interested in. I would focus on telemetry/reporting plus anything related to ad craziness. Not bothered enough to do it though as I do not need more confirmation they are lost in traditional monetization ideas. Rest is zzzzz.