Thank you Rob!
Works great even with FF older than 57.

For people who, do not understand much about javascript,css syntax or even html, but know how an ip and web-page address looks like, it is till useful to look at an extension source code.
In that regard would be very useful if you could made somehow a filter(search) that finds web-pages and ip adresses inside the source code of the extension. Now, someone can manually find them either by looking to all the lines in all the files(which is too much for most of us), or by searching "something", like '!.com' and then going with "next" trough all the files, but there are a lot of finds that are not addresses, like for ex. ".comments". The idea is to hit some desirably "search addresses" and to find only ip and web adresses, so then we could jump to each of them inside the code and decide next if further investigation or not is needed. Well, I was dreaming with my eyes open, but anyway, even if you wont do what I was trying to explain, you already have done a good job for over simplifying the reading of the source code.

Someone mentioned earlier in a comment, something about firewall for Firefox....well, uMatrix is the closest FF extension to a real firewall, that I know of. Check it out: https://cup.aihedy.com/en-US/firefox/addon/umatrix/

Cheers!