# Set up to build Firefox Developer Tools
These are the steps we're going to look at:
* [Getting the code](#getting-the-code)
* [Building and running locally](#building-and-running-locally)
* [Artifact builds](#building-even-faster-with-artifact-builds) for even faster builds
* [Maybe you don't even need to build](#maybe-you-dont-even-need-to-build)
## Getting the code
The code is officially hosted on [a Mercurial repository](https://hg.mozilla.org/mozilla-central/). It takes a long time to clone, because the repository is **B I G**. So be prepared to be patient.
hg clone http://hg.mozilla.org/mozilla-central
## Building and running locally
Fortunately, the Firefox team has made a very good job of automating the building process with bootstrap scripts and putting [documentation](https://developer.mozilla.org/En/Simple_Firefox_build) together.
The very first time you are building Firefox, run:
After that first successful build, you can just run:
If your system needs additional dependencies installed (for example, Python, or a compiler, etc), various diagnostic messages will be printed to your screen. Follow their advice and try building again.
Building also takes a long time (specially on slow computers).
**Note:** if using Windows, you might need to type `mach build` instead (without the `./`).
### Running your own compiled version of Firefox
To run the Firefox you just compiled:
This will run using an empty temporary profile which is discarded when you close the browser. We will look more into [persistent development profiles later](./development-profiles.md).
Suppose you pulled the latest changes from the remote repository (or made your own changes) and want to build again.
You can ask the `mach` script to build only changed files:
./mach build faster
This should be faster (a matter of seconds).
Sometimes, if you haven't updated in a while, you'll be told that you need to *clobber*, or basically delete precompiled stuff and start from scratch, because there are too many changes. The way to do it is:
### Building even faster with artifact builds
If you are *not* going to modify any C/C++ code (which is rare when working on DevTools), you can use artifact builds. This method downloads prebuilt binary components, and then the build process becomes faster.
Create a file on the root of the repository, called `mozconfig`, with the following content:
# Automatically download and use compiled C++ components:
# Write build artifacts to:
And then you can follow the normal build process again (only *faster*!)
On MacOS you might want to use `MOZ_OBJDIR=./objdir-frontend.noindex` instead. Using the `.noindex` file extension prevents the Spotlight from indexing your `objdir`, which is slow.
For more information on aspects such as technical limitations of artifact builds, read the [Artifact Builds](https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Artifact_builds) page.
## Maybe you don't even need to build
Instead, what you need to do is to save the files you modified, quit Firefox, and reopen it again to use the recently saved files.
# 1. Build
# 2. Run
# 3. you try out things in the browser that opens
# 4. fully close the browser, e.g. ⌘Q in MacOS
# 5. edit JS files on the `devtools` folder, save
# 6. Back to step 2!
While not as fast as the average "save file and reload the website" *web development workflow*, or newer workflows such as React's reloading, this can still be quite fast.
And certainly faster than building each time, even with artifact builds.