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

Setup

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. Install a specific cbindgen

Install cbindgen version 0.24.3 in an isolated location so it does not interfere with other trees that need a newer version:

cargo install cbindgen --version 0.24.3 --root ~/.local/cbindgen-0.24.3

3. mozconfig

ac_add_options --with-libclang-path=/usr/lib/llvm-16/lib
export CBINDGEN=/home/YOU/.local/cbindgen-0.24.3/bin/cbindgen

--with-libclang-path tells the build system which libclang to hand to bindgen. CBINDGEN overrides the cbindgen binary for this tree only; the system-wide cbindgen (used by other trees) is left untouched.

llvm-16 must be installed (apt install llvm-16 libclang-16-dev or equivalent).

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.