Hi v.tank. The first step to get started is to check out the Firefox codebase. Firefox is using Mercurial, which is an alternative to Git. We have a "getting started" guide here: https://docs.firefox-dev.tools/getting-started/build.html
One bit of advice: you will only need "artifact builds", which are much faster than full builds, for this work. So when setting up, before building for the first time using the
./mach build command, I highly recommend following the steps in the Artifact Builds section.
The whole process is a bit long, you may need a few hours to get things right. If at any point you are stuck, you can ask for help on the Firefox DevTools slack at https://devtools-html-slack.herokuapp.com/
Once setup, most of the files you will need to change are in the
devtools/client/locales/en-US directory. You can see its current content online here:
You can use any code editor to edit them (VS Code, Sublime Text, any big IDE, etc.).
I think we should break the work by DevTools tool (also called a "panel"). Maybe start with the Console?
So the first step, after you've successfully built Firefox locally, would be to edit a few of the reported strings in
devtools/client/locales/en-US/webconsole.properties, build and run Firefox again (
./mach build && ./mach run), and check in the Console that the result looks okay.
Does this look manageable? I know the initial setup can be difficult, but feel free to ask questions here or on Slack!