Working with ESR115

Firefox 115 was released in mid-2023 and is going to be supported for some time for Windows 7 users.

Building it on a modern Linux system (that a lot of Mozilla developer use) requires working around several toolchain version mismatches. None of the fixes require patching Firefox source.

Additionally, pushing to try doesn’t work out of the box.

This document explains a setup that allows verifying patches locally and pushing to try, it is Linux only but might work on other OSes.

We’ll be working in a separate checkout in this document, since a special mozconfig and rust-toolchain.toml files are required.

Required toolchain versions

Tool	Required	Reason
Rust	1.76.0	Newer versions break bindgen-generated bindings
cbindgen	0.24.3	0.29 is too strict: rejects duplicate TOML keys and treats `try` as a reserved keyword
libclang	16 (not 18)	bindgen 0.64 (used by gecko-profiler) generates incomplete bindings with clang 18
Python	<= 3.12	Python 3.13 removes `cgi`, `pipes` and `pathlib.os` that build tools depend on

Setup

If your system Python is 3.13 or later, prefix ./mach invocations with uv run -p 3.12 (or uv run -p 3.11).

1. Pin the Rust toolchain for the tree

Create rust-toolchain.toml at the root of the Firefox 115 checkout:

[toolchain]
channel = "1.76.0"

Install the toolchain if not already present needed:

rustup toolchain install 1.76.0

2. mozconfig

ac_add_options --enable-bootstrap

--enable-bootstrap downloads compatible cbindgen and libclang from the Taskcluster toolchain index into ~/.mozbuild/ automatically.

Cross-compiling for Windows (from Linux)

This is useful for backporting patches to Windows 7 without needing a Windows machine. The build system supports cross-compiling with clang-cl (MSVC ABI, Tier-1).

1. Install required tools

sudo apt install msitools
rustup target add x86_64-pc-windows-msvc

msitools provides msiextract, which is required to unpack the Windows SDK .msi packages during the bootstrap step.

2. mozconfig for Windows cross-compile

Use a separate mozconfig (e.g. mozconfig.win64) or replace the Linux one when working on a Windows-only issue:

ac_add_options --target=x86_64-pc-windows-msvc
ac_add_options --enable-bootstrap

--enable-bootstrap downloads the Visual Studio sysroot (headers, libs, Windows SDK) from the Taskcluster toolchain index into ~/.mozbuild/ and selects clang-cl and lld-link automatically. No manual SDK installation is needed.

Pushing to try

From a regular checkout (from https://github.com/mozilla-firefox/firefox), esr115 can be checked out like so:

git checkout esr115

and will track upstream when pulled.

To push to try, it is required to have git-cinnabar in path (that ./mach vcs-setup can install), and to run:

git -c cinnabar.graft=https://github.com/mozilla-firefox/firefox fetch hg::https://hg.mozilla.org/mozilla-unified

./mach try fuzzy will then produce some warnings, but work (it is expected that it takes some time). The push go via SSH, so a level 1 account is necessary.