Some rust things fail:

error: could not compile `encoding_rs`.

Caused by:
  process didn't exit successfully: `CARGO=/usr/bin/cargo CARGO_CRATE_NAME=encoding_rs CARGO_MANIFEST_DIR=/build/js78/src/firefox-78.7.0/third_party/rust/encoding_rs CARGO_PKG_AUTHORS='Henri Sivonen <hsivonen@hsivonen.fi>' CARGO_PKG_DESCRIPTION='A Gecko-oriented implementation of the Encoding Standard' CARGO_PKG_HOMEPAGE='https://docs.rs/encoding_rs/' CARGO_PKG_LICENSE=MIT/Apache-2.0 CARGO_PKG_LICENSE_FILE='' CARGO_PKG_NAME=encoding_rs CARGO_PKG_REPOSITORY='https://github.com/hsivonen/encoding_rs' CARGO_PKG_VERSION=0.8.22 CARGO_PKG_VERSION_MAJOR=0 CARGO_PKG_VERSION_MINOR=8 CARGO_PKG_VERSION_PATCH=22 CARGO_PKG_VERSION_PRE='' LD_LIBRARY_PATH='/build/js78/src/firefox-78.7.0/obj/release/deps:/usr/lib' OUT_DIR=/build/js78/src/firefox-78.7.0/obj/i686-unknown-linux-gnu/release/build/encoding_rs-1e3d846dc5da0b4e/out RUSTC_BOOTSTRAP=1 /usr/bin/rustc --crate-name encoding_rs /build/js78/src/firefox-78.7.0/third_party/rust/encoding_rs/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -C opt-level=2 -C panic=abort -C embed-bitcode=no --cfg 'feature="packed_simd"' --cfg 'feature="simd-accel"' -C metadata=825b3c8ce2e0cf97 -C extra-filename=-825b3c8ce2e0cf97 --out-dir /build/js78/src/firefox-78.7.0/obj/i686-unknown-linux-gnu/release/deps --target i686-unknown-linux-gnu -C linker=/build/js78/src/firefox-78.7.0/build/cargo-linker -L dependency=/build/js78/src/firefox-78.7.0/obj/i686-unknown-linux-gnu/release/deps -L dependency=/build/js78/src/firefox-78.7.0/obj/release/deps --extern cfg_if=/build/

So this boils down to fixing Rust for 1.49 first and hope it will build with that version
(or alternatively we could also try rust148 first).

ld.lld: error: cannot open /usr/lib/clang/11.0.0/lib/linux/libclang_rt.profile-i386.a: No such file or directory
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)
make[3]: *** [/build/js78/src/firefox-78.7.0/config/rules.mk:606: libmozjs-78.so] Error 1
make[3]: *** Waiting for unfinished jobs....

using clang target –target=i686-unknown-linux-gnu

Interesting:

ls /usr/lib/clang/

reveils a 11.0.0 and a 11.0.1

the include files are in 11.0.0, the libraries in 11.0.1 and the builds
search for the libraries in 11.0.0:

ld.lld: error: cannot open /usr/lib/clang/11.0.0/lib/linux/libclang_rt.profile-i386.a: No such file or directory

This sounds like a misbuild of clang..

Let's blindly reschedule clang and llvm builds.

Now check fails in some Python test code. To me it's not clear whether this
is a general Python issue or just this test framework.

==> Starting check()...
make: Entering directory '/build/js78/src/firefox-78.7.0/obj/js/src'
../../dist/bin/run-mozilla.sh /build/js78/src/firefox-78.7.0/obj/_virtualenvs/init_py3/bin/python -u /build/js78/src/firefox-78.7.0/js/src/tests/jstests.py \
        --no-progress --format=automation --timeout 300 \
        --format=none --exclude-random --wpt=disabled \
        ../../dist/bin/js
Traceback (most recent call last):
  File "/build/js78/src/firefox-78.7.0/js/src/tests/jstests.py", line 676, in <module>
    sys.exit(main())
  File "/build/js78/src/firefox-78.7.0/js/src/tests/jstests.py", line 623, in main
    for out in run_all_tests(test_gen, prefix, results.pb, options):
  File "/build/js78/src/firefox-78.7.0/js/src/tests/lib/tasks_unix.py", line 236, in run_all_tests
    tests = list(tests)
  File "/build/js78/src/firefox-78.7.0/js/src/tests/jstests.py", line 548, in <genexpr>
    test_gen = (test for test in test_gen for i in range(options.repeat))
  File "/build/js78/src/firefox-78.7.0/js/src/tests/jstests.py", line 545, in <genexpr>
    test_gen = (_ for _ in test_gen if not _.slow)
  File "/build/js78/src/firefox-78.7.0/js/src/tests/jstests.py", line 538, in <genexpr>
    test_gen = (_ for _ in test_gen if not _.random)
  File "/build/js78/src/firefox-78.7.0/js/src/tests/lib/manifest.py", line 533, in load_reftests
    _parse_test_header(fullpath, testcase, xul_tester)
  File "/build/js78/src/firefox-78.7.0/js/src/tests/lib/manifest.py", line 407, in _parse_test_header
    _parse_one(testcase, matches.group(2), xul_tester)
  File "/build/js78/src/firefox-78.7.0/js/src/tests/lib/manifest.py", line 221, in _parse_one
    if xul_tester.test(cond, testcase.options):
  File "/build/js78/src/firefox-78.7.0/js/src/tests/lib/manifest.py", line 108, in test
    return self._test_local(cond, options=options)
  File "/build/js78/src/firefox-78.7.0/js/src/tests/lib/manifest.py", line 178, in _test_local
    raise Exception("Failed to test XUL condition {!r};"
Exception: Failed to test XUL condition "!this.hasOwnProperty('foobar')||outro()"; output was '', stderr was ''
make: *** [Makefile:63: check-jstests] Error 1
make: Leaving directory '/build/js78/src/firefox-78.7.0/obj/js/src'
==> ERROR: A failure occurred in check().
    Aborting...
==> ERROR: Build failed, check /var/lib/archbuild/staging-pentium4/abaumann/build

/usr/bin/cargo rustc  --release --manifest-path /build/js78/src/firefox-78.7.1/js/src/rust/Cargo.toml -vv --lib --target=i686-unknown-linux-gnu  --  -Clto
error: failed to parse manifest at `/build/js78/src/firefox-78.7.1/Cargo.toml`

Caused by:
  dependency (nix) specification is ambiguous. Only one of `branch`, `tag` or `rev` is allowed.
make[3]: *** [/build/js78/src/firefox-78.7.1/config/makefiles/rust.mk:310: force-cargo-library-build] Error 101

We have a rust version which cannot compile older versions of js now?

Let's try to pin rust to 1.49 or rust 148 (on pentium4 as we have rust 1.50 there already).
So js78 compiles on i686 because rust 1.50 failed to build.
This is software development in 2021 at its best!