Inspired by the repo with QC35-2 firmwares by avicoder.
It seems like the latest firmware, v3.0.3, for the Bose Quiet Confort 35, first version (QC35-1) has a major problem that makes the active noise cancelling worse, Bose denies it, users aren't able to (officially) downgrade to the previous version, v2.5.5.
Fortunally, u/akumanara posted an unofficial firmware downgrade guide for QC35-2 on Reddit, also affected. It turns out that the QC35-1 and 2 have different codenames and firmware files. The QC35-2 has the codename "baywolf" and QC35-1 has the codename "wolfcastle", as pointed out by u/TeRmRaN19. With this information the firmware files for QC35-1 could be downloaded and the guide can be adapted as needed for proper QC35-1 on v3.0.3 firmware (untested by me, as I was lucky enough to abort my firmware upgrade when I saw the forum posts about it).
The files were obtained by inspecting this URLs as of 07/26/2019:
https://downloads.bose.com/ced/PRE_RRA/wolfcastle/index.xml (older, stable, v2.5.5 firmware)
https://downloads.bose.com/ced/wolfcastle/index.xml (latest, bugged, v3.0.3 firmware)
So here are the files as a backup if they disappear.