Wa-alaykum al-salam wa-rahmat Allah.
Dear brother,
The book
Musnad al-Firdaws (مسند الفردوس) by
[Abū Manṣūr] al-Daylamī (Jr) is a
lost work: it is
available only partially (in manuscript form), and has, hence,
never been published, to my knowledge.
However,
Ibn Ḥajar al-ʿAsqalānī has authored a book titled
Zahr al-Firdaws (زهر الفردوس), also known as
al-Gharā’ib al-Multaqaṭah min Musnad al-Firdaws (الغرائب الملتقطة من مسند الفردوس), in which he has narrated (some) ḥadīths from
Musnad al-Firdaws of [Abū Manṣūr] al-Daylamī (Jr).
You may download
Zahr al-Firdaws from here:
(1) الغرائب الملتقطة من مسند الفردوس مما ليس في الكتب المشهورة - مخطوط: manuscript transcript in BOK format:
https://shamela.ws/rep.php/book/522 [BOK].
(2) الغرائب الملتقطة من مسند الفردوس = زهر الفردوس: published for the first time by Jamʿiyyah Dār al-Birr (جمعية دار البر), Dubai, in 1439/2018:
https://www.almeshkat.net/book/14654 [PDF; BOK].
To open BOK (.bok) file,
download, extract, and run/install the following
“empty/emptied (مفرغ)” version of the
Shamela Islamic Library (المكتبة الشاملة) v3.64:
https://www.almeshkat.net/book/2288.
Hope you found it helpful.
Wal-salam alaykum wa-rahmat Allah.