The core feature of a browser is browsing. Web dev tools are secondary. To a regular user who is not dev the removal of RSS is a loss. JSON is secondary. Firefox would make a mistake if they prioritized Dev tools over end user features.

How many regular users actually use RSS? To be honest, I don't think most devs use RSS, much less non-devs.

Mozilla has a finite budget to allocate to features. Maybe they looked at usage rates and decided that RSS just wasn't an important feature, or other features were more important.

How many regular users use JSON or other development tools?

Very few, but I bet there's an order of magnitude more developers who regularly view JSON in a browser than RSS in a browser (without extensions).

Believe it or not JSON is not the only data transfer format used on the web. Some web services even spit out XML, crazy I know.

There is a separate "Firefox Developer Edition" for that.

I'm strongly in favor of anything that makes it easier for non developers to "see behind the scenes" into computers, so while developer edition is nice and all I don't consider it a strong reason not to include development tools in regular firefox.