Is it possible to arbitrarily change the html (including scripts) code of a page loaded in Firefox on the fly?

Question

JavaScript

Hello. Is it possible in Firefox to change the code of the loaded page on the fly, as it was in the old opera before switching to the new engine. Those. there you could just open the source of the page and change anything, click "save changes" and it was immediately displayed on the page. I tried firebug, but did not find there the possibility of free editing of the entire page. You can edit the body or individual elements, but, for example, I could not insert a new script block in the head of the page. In general, is it possible to organize this?

Answer 1

In Firebug, in the HTML section, there is a button at the top - "Edit"
After clicking, the code is replaced by an editor where you can write anything you want.
Pressing "Edit" again saves the changes.

Answer 2

Double click on any element in Firebug

Answer 3

Well, paste from the console with javascript

Answer 4

If you need to execute your script on the page, then see gracemonkey: https://addons.mozilla.org/ru/firefox/addon/grease...
Then your code will be inserted and executed, and you can do everything with the page in it within allowed by firefox. For example, change the width of all or individual input fields.