www/firefox and mail/thunderbird fail to build due to missing makefile variable

Hi everyone,

both www/firefox and mail/thunderbird fail to build on a freshly 
installed 14.2-STABLE/amd64 with a similar error:

[firefox-133.0.3,2]
----8<----
[...]
gmake[2]: Leaving directory '/usr/ports/www/firefox/work/.build'
gmake[1]: *** 
[/usr/ports/www/firefox/work/firefox-133.0.3/config/recurse.mk:34: 
compile] Error 2
gmake[1]: Leaving directory '/usr/ports/www/firefox/work/.build'
gmake: *** 
[/usr/ports/www/firefox/work/firefox-133.0.3/config/rules.mk:359: all] 
Error 2
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/www/firefox
*** Error code 1

Stop.
make: stopped in /usr/ports/www/firefox
---->8----

[thunderbird-128.6.0]:
----8<----
[...]
gmake[2]: Leaving directory '/usr/ports/mail/thunderbird/work/.build'
gmake[1]: *** 
[/usr/ports/mail/thunderbird/work/thunderbird-128.6.0/config/recurse.mk:34: 
compile] Error 2
gmake[1]: Leaving directory '/usr/ports/mail/thunderbird/work/.build'
gmake: *** 
[/usr/ports/mail/thunderbird/work/thunderbird-128.6.0/config/rules.mk:361: 
all] Error 2
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/mail/thunderbird
*** Error code 1

Stop.
make: stopped in /usr/ports/mail/thunderbird
---->8----

Looking at the last reported error in rules.mk, it seems as if in both 
cases a variable is not set correctly somewhere:
----8<----
[...]
# The root makefile doesn't want to do a plain export/libs, because
# of the tiers and because of libxul. Suppress the default rules in favor
# of something else. Makefiles which use this var *must* provide a sensible
# default rule before including rules.mk
default all::
         $(foreach tier,$(TIERS),$(call SUBMAKE,$(tier)))
[...]
---->8----

Before trying to chase this any further, has anyone else encountered 
this already?

Best
Philipp