firefox: Missing ICU patches break functionality of Intl/Temporal JavaScript built-ins

[anba]

Nixpkgs version

• Unstable (26.05)

Describe the bug

Original bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=2012175

#478573 updated the Firefox package to ICU 78, but because the "buildMozillaMach" package sets --with-system-icu, some ICU 78 patches needed for Firefox aren't used.

The following patches from https://github.com/mozilla-firefox/firefox/tree/main/intl/icu-patches are needed:

1. https://github.com/mozilla-firefox/firefox/blob/main/intl/icu-patches/bug-2000225-ICU-23262-missing-resource-error-for-iso8601-era.diff to ensure the "iso8601" calendar can be used for the Intl.DateTimeFormat built-in. (https://unicode-org.atlassian.net/browse/ICU-23262)
2. https://github.com/mozilla-firefox/firefox/blob/main/intl/icu-patches/bug-2000225-ICU-23264-increase-measure-unit-capacity.diff to ensure volume units can be used by Intl.NumberFormat. (https://unicode-org.atlassian.net/browse/ICU-23264)
3. https://github.com/mozilla-firefox/firefox/blob/main/intl/icu-patches/bug-2002735-ICU-23277-coptic-single-era.diff to ensure the "coptic" calendar is correctly formatted by Intl.DateTimeFormat. (https://unicode-org.atlassian.net/browse/ICU-23277)
4. https://github.com/mozilla-firefox/firefox/blob/main/intl/icu-patches/bug-2010411-patch-cldr-hv-en-root.diff to fix web-compat issues with old versions of the "date-fns" JavaScript library (https://unicode-org.atlassian.net/browse/CLDR-19205). (The Firefox patch has the minimal changes to fix the web-compat issues, upstream CLDR applied more changes in CLDR-19205 Remove literal 'h' from en availableFormats for Hv unicode-org/cldr#5309 and later in CLDR-19205 Remove literal 'h' or equiv added to Hv availableFmts in CLDR48 unicode-org/cldr#5320.)
5. And either https://github.com/mozilla-firefox/firefox/blob/09295ebd1dfb7ab08e8b8ad94f0a7302f5ceebe6/intl/icu-patches/bug-2002997-ICU-23278-metazone-with-offset.diff or update to ICU 78.2. This fixes some time zone issues for "Europe/Dublin" and "Africa/Windhoek", observable using either Intl.DateTimeFormat or the Temporal JS built-in. (https://unicode-org.atlassian.net/browse/ICU-23278.)

Alternatively use the ICU version included in Firefox, which also fixes other bugs, like for example https://bugzilla.mozilla.org/show_bug.cgi?id=1999917.

Steps to reproduce

Evaluate new Intl.NumberFormat("en-GB", { style: 'unit', unit: 'liter' }).format(1) in the Firefox dev console.

(This is the test case from https://bugzilla.mozilla.org/show_bug.cgi?id=2012175, caused by https://unicode-org.atlassian.net/browse/ICU-23264.)

Expected behaviour

The test case should print the string "1 l" in the Firefox dev console, but instead throws Uncaught Error: internal error while computing Intl data.

Screenshots

No response

Relevant log output

Additional context

No response

System metadata

From https://bugzilla.mozilla.org/show_bug.cgi?id=2012175#c3:

Operating System: NixOS 26.05
KDE Plasma Version: 6.5.5
KDE Frameworks Version: 6.22.0
Qt Version: 6.10.1
Kernel Version: 6.18.6 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 5800X 8-Core Processor
Memory: 32 GiB of RAM (31,3 GiB usable)
Graphics Processor: AMD Radeon RX 7900 XT
Manufacturer: Micro-Star International Co., Ltd.
Product Name: MS-7C84
System Version: 1.0

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@mweinelt

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