Let's say I open a webpage with some Unicode characters, say, Cyrillic, in the address like this:
When I try to copy it from the address bar somewhere else, it becomes unreadable rubbish:
I guess this is for compatibility. However for readability I want to copy it straight away with proper Unicode characters.
What and how should I tweak to make that possible?
Edit: The answers are good, but they are hacky. Are there any settings in about:config?
The setting was replaced with browser.urlbar.decodeURLsOnCopy in Firefox 53+. Set it to true.
For copying in Firefox on Android see https://android.stackexchange.com/q/224951/17612
Try setting the network.standard-url.escape-utf8 option via about:config.
Possible values and their effects:
true - Escape UTF-8 URLs. (Default)false - Do not escape UTF-8 URLs. 
Yes, browsers do that -- it's URL encoding and actually a quite useful thing.
I see two choices
Copy only a fragment of the URL, it will not be URL encoded.
Decode the URL encoding. In Unix, you can open a terminal, and type
echo -n -e "$(echo http://ru.wikipedia.org/wiki/%D0%A4%D1%83%D0%BD%D0%BA%D1%86%D0%B8%DB0%D0%BB%D1%8C%D0%BD%D0%B0%D1%8F_%D0%B7%D0%B0%D0%BA%D1%80%D0%B5%D0%BF%D0%BB%D1%91%D0%BD%D0%BD%D0%BE%D1%81%D1%82%D1%8C | sed 's/+/ /g;s/%\(..\)/\\x\1/g;')"
It's not rubbish, it's URL encoded. However here's the trick:
You can edit the URL in the address bar (by adding and then removing a character), and the copy-pasting should preserve the encoding.
Use this key sequence to copy UTF-8 URL:
Ctrl-L, Right-arrow(or Left/Home/End), Space(or a letter/symbol), Backspace, Ctrl-L, Ctrl-C
