sudo pacman -Syu
[sudo] password for .../...:
:: Synchronizing package databases...
system is up to date
world is up to date
galaxy is up to date
omniverse is up to date
:: Starting full system upgrade...
.../...
resolving dependencies...
warning: cannot resolve "libvpx.so=11-64", a dependency of "thunderbird"
warning: cannot resolve "libvpx.so=11-64", a dependency of "thunderbird"
:: The following package cannot be upgraded due to unresolvable dependencies:
thunderbird
The problem is now resolved.
Thanks to the package maintainers! 8)