One thing that is really handy for development is that you can set up any container to use a proxy server. I use that a lot to reduce the sheer amount of crap that would get sent to the proxy if it were enabled at the system level or in the regular browser network settings. It really keeps the noise down.
The Multi-Account Containers extension is great for this. Each container keeps its own context, so you can be logged in to the same service twice (or more) in tabs in one window. Can set it up so that some sites will always use a certain container, or that sites in a container will always use a proxy. That is EXTREMELY useful to me.
https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/