Hello, Gentoo.

I'm trying to do

    # emerge -a @preserved-rebuild

..  For this purpose, I created a temporary repository, filling it with
ancient ebuilds recovered from /var/db/pkg.

It shows me 5 packages to be merged, among them being

    [ebuild   R    ] dev-lang/spidermonkey-78.15.0 [78.15.0]

..  When I answer the "are you sure?" prompt with Yes, spidermonkey fails
to build with this error message:

Emerging (1 of 5) dev-lang/spidermonkey-78.15.0::localrepo

!!! Fetched file: firefox-78.15.0esr.source.tar.xz VERIFY FAILED!
!!! Reason: Insufficient data for checksum verification
!!! Got:
!!! Expected: BLAKE2B BLAKE2S MD5 RMD160 SHA1 SHA256 SHA3_256 SHA3_512
SHA512 WHIRLPOOL
 * Fetch failed for 'dev-lang/spidermonkey-78.15.0', Log file:
 *  '/var/tmp/portage/dev-lang/spidermonkey-78.15.0/temp/build.log'

It would appear that the sources are no longer available at Firefox, and
I no longer have a copy in /usr/portage/distfiles.

Why is portage trying to build this obsolete version?  I surely will just
unmerge it as soon as my system is back in order.

Most importantly, how can I free up my system so that I can proceed with
the pending profile update?  I appear to be stuck.  I hope I won't have
to reinstall gentoo.