13.4 compiles firefox functionally different

Hi,

 after upgrading to 13.4-RC2-p1, I recompiled all ports.

Today I found my firefox can no longer render a certain webpage;
the output is garbled as if the CSS were missing/defective.
Here is details and picture:
https://forums.freebsd.org/threads/firefox-displays-unintellegible-garbage-only-on-freebsd.94963/post-671994

The firefox version is -esr-115.15.0,1, from 2024Q3 as of a
week ago.

Investigation showed:
 * firefox from package as of today (130.something) works well.
 * firefox-esr from package as of today (128.something) works
   well.
 * firefox-esr from ports as of today (128.2.0esr) freshly compiled
   on 13.4-RC2-p1 also works well (prereqs still from last week)
 * firefox-esr 115.14.0_1,1 locally compiled a few weeks ago on
   Rel. 13.3 also works well.

So I did verify: I restarted a Rel.13.3-p5, fetched all
prereqisite ports from backup (so these were also compiled on 13.3),
and then compiled firefox-esr-115.15.0,1, the very same that now
renders garbage, using the very same ports tree, with OS running
13.3-p5. And that one now works!

So bottomline is:
firefox-esr 115.15.0,1 - the one that was in 2024Q3 until about a
week ago - works correct when compiled on 13.3 (along with all it's
prereq ports), but renders garbage for a certain webpage when
compiled on 13.4-RC2 (along with all it's prereq ports).

I don't like this, because now the question is: what else might 13.4
compile so that it suddenly functions differently?


(I glanced thru PR 277021 - but it doesn't seem to mention anything
about webpages displaying broken, or any functional difference)

Any idea how this might come to happen, anybody?