www-client/firefox depends on llvm <= 12. It should work with llvm 13.
Likewise for dev-lang/spidermonkey
Firefox and Spidermonkey do work with llvm/clang/lld slot 13. I have tested this by modifying the ebuilds.
Used LLVM slot must have matching rust version using same slot. That wasn't the case until rust-1.56. However, firefox currently doesn't build against rust-1.56 so we will not allow LLVM:13 yet for better user experience.
The bug has been referenced in the following commit(s): https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=ba9cb8fb42273470a6eda4dec9e5fb72a0615214 commit ba9cb8fb42273470a6eda4dec9e5fb72a0615214 Author: Thomas Deutschmann <whissi@gentoo.org> AuthorDate: 2021-10-22 23:53:25 +0000 Commit: Thomas Deutschmann <whissi@gentoo.org> CommitDate: 2021-10-23 00:33:37 +0000 mail-client/thunderbird: update patch set - Unbreak rust-1.56 - Allow building with LLVM 13 - Add Python 3.10 support Bug: https://bugs.gentoo.org/818016 Bug: https://bugs.gentoo.org/819486 Package-Manager: Portage-3.0.28, Repoman-3.0.3 Signed-off-by: Thomas Deutschmann <whissi@gentoo.org> mail-client/thunderbird/Manifest | 2 +- mail-client/thunderbird/thunderbird-78.14.0.ebuild | 14 +++++++++++--- 2 files changed, 12 insertions(+), 4 deletions(-) https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=f5b8ac821e3a3cbe8c9a7edfd4109e3ddaf19f19 commit f5b8ac821e3a3cbe8c9a7edfd4109e3ddaf19f19 Author: Thomas Deutschmann <whissi@gentoo.org> AuthorDate: 2021-10-22 23:48:33 +0000 Commit: Thomas Deutschmann <whissi@gentoo.org> CommitDate: 2021-10-23 00:33:35 +0000 dev-lang/spidermonkey: update patch set - Unbreak rust-1.56 - Allow building with LLVM 13 - Add Python 3.10 support Bug: https://bugs.gentoo.org/818016 Bug: https://bugs.gentoo.org/819486 Package-Manager: Portage-3.0.28, Repoman-3.0.3 Signed-off-by: Thomas Deutschmann <whissi@gentoo.org> dev-lang/spidermonkey/Manifest | 2 +- dev-lang/spidermonkey/spidermonkey-78.15.0.ebuild | 13 ++++++++++--- 2 files changed, 11 insertions(+), 4 deletions(-) https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=57093afe2fa8a6641603e53a9ccc6349f114bc57 commit 57093afe2fa8a6641603e53a9ccc6349f114bc57 Author: Thomas Deutschmann <whissi@gentoo.org> AuthorDate: 2021-10-22 23:46:49 +0000 Commit: Thomas Deutschmann <whissi@gentoo.org> CommitDate: 2021-10-23 00:33:32 +0000 www-client/firefox: update patch set - Unbreak rust-1.56 - Allow building with LLVM 13 - Add Python 3.10 support Bug: https://bugs.gentoo.org/818016 Bug: https://bugs.gentoo.org/819486 Package-Manager: Portage-3.0.28, Repoman-3.0.3 Signed-off-by: Thomas Deutschmann <whissi@gentoo.org> www-client/firefox/Manifest | 4 ++-- www-client/firefox/firefox-78.15.0.ebuild | 14 +++++++++++--- www-client/firefox/firefox-93.0.ebuild | 12 ++++++++++-- 3 files changed, 23 insertions(+), 7 deletions(-)