I try to use this: https://addons.mozilla.org/firefox/addon/firebug/ but it isn't updated for weeks.
I want to use Firefox, as it's open source. Is there a Firebug alternative? Or what's the easiest way to add Firebug to the latest Firefox?
Firebug is officially discontinued for several years now. E.g. there's an announcement from 2015.
The main reasons for Firebug not being continued is that internal changes in Firefox for better stability and performance, namely separated processes for the browser's UI and the web contents and restricting add-on support to cross-browser compatible WebExtensions, would have required to rewrite the majority of its code and would have it's feature set due to a limited API.
Mozilla suggests to use the Firefox Developer Tools instead, which are built into Firefox and can be opened via F12, Ctrl+Shift+I or via the Firefox Menu > Web Developer > Toggle Tools.
Note that like Firefox, those tools are open source. Their code is partly hosted on a Mozilla repository and partly on a GitHub repository.