$ yt-dlp -vU --cookies-from-browser firefox -F https://www.youtube.com/watch?v=IM4zZchluX0
[debug] Command-line config: ['-vU', '--cookies-from-browser', 'firefox', '-F', 'https://www.youtube.com/watch?v=IM4zZchluX0']
[debug] Encodings: locale UTF-8, fs utf-8, pref UTF-8, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version stable.05 from yt-dlp/yt-dlp [50136eeeb]
[debug] Lazy loading extractors is disabled
[debug] Python 3.13.7 (CPython x86_64 64bit) - Linux-6.16.8-200.fc42.x86_64-x86_64-with-glibc2.41 (OpenSSL 3.2.4 11 Feb 2025, glibc 2.41)
[debug] exe versions: ffmpeg 7.1.1 (fdk,setts), ffprobe 7.1.1
[debug] Optional libraries: Cryptodome-3.23.0, brotli-1.1.0, certifi-2025.07.09, mutagen-1.47.0, requests-2.32.4, sqlite3-3.47.2, urllib3-2.3.0, websockets-14.1
[debug] Proxy map: {}
Extracting cookies from firefox
[debug] Extracting cookies from: "/home/cpn/.mozilla/firefox/2c6ckpp7.default-release/cookies.sqlite"
Extracted 1795 cookies from firefox
[debug] Request Handlers: urllib, requests, websockets
[debug] Plugin directories: none
[debug] Loaded 1842 extractors
[debug] Fetching release info: https://api.github.com/repos/yt-dlp/yt-dlp/releases/latest
[debug] Downloading _update_spec from https://github.com/yt-dlp/yt-dlp/releases/latest/download/_update_spec
Current version: stable.05 from yt-dlp/yt-dlp
Latest version: stable.26 from yt-dlp/yt-dlp
ERROR: You installed yt-dlp from a manual build or with a package manager; Use that to update
[debug] [youtube] Found YouTube account cookies
[debug] [youtube] [pot] PO Token Providers: none
[debug] [youtube] [pot] PO Token Cache Providers: memory
[debug] [youtube] [pot] PO Token Cache Spec Providers: webpo
[youtube] Extracting URL: https://www.youtube.com/watch?v=IM4zZchluX0
[youtube] IM4zZchluX0: Downloading webpage
[debug] [youtube] Detected YouTube Premium subscription
[youtube] IM4zZchluX0: Downloading tv client config
[debug] Loading youtube-sts.a61444a1-main from cache
[youtube] IM4zZchluX0: Downloading tv player API JSON
[youtube] IM4zZchluX0: Downloading web creator player API JSON
[youtube] IM4zZchluX0: Downloading player a61444a1-main
[debug] [youtube] Initial search was unable to find nsig function name
        player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
WARNING: [youtube] Falling back to generic n function search
         player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
WARNING: [youtube] IM4zZchluX0: nsig extraction failed: Some formats may be missing
         n = b4lDjL9bja6Du5wv ; player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
         Please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U
[debug] [youtube] Unable to extract nsig function code (caused by RegexNotFoundError('Unable to extract \x1b[0;94mInitial JS player n function name\x1b[0m; please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U')); please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U
WARNING: [youtube] IM4zZchluX0: nsig extraction failed: Some formats may be missing
         n = FfXFnHa1SiYrBjdn ; player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
         Please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U
WARNING: [youtube] IM4zZchluX0: nsig extraction failed: Some formats may be missing
         n = GnKetpU4OOhCH3Ze ; player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
         Please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U
WARNING: [youtube] IM4zZchluX0: nsig extraction failed: Some formats may be missing
         n = GKnxLvXfWwWs0NdM ; player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
         Please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U
WARNING: [youtube] IM4zZchluX0: nsig extraction failed: Some formats may be missing
         n = djTHMPf8cXPsc9Uo ; player = https://www.youtube.com/s/player/a61444a1/player_ias.vflset/en_US/base.js
         Please report this issue on  https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template. Confirm you are on the latest version using  yt-dlp -U
WARNING: [youtube] IM4zZchluX0: Some web client https formats have been skipped as they are missing a url. YouTube is forcing SABR streaming for this client. See  https://github.com/yt-dlp/yt-dlp/issues/12482  for more details
WARNING: Only images are available for download. use --list-formats to see them
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, vcodec, channels, acodec, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, quality, res, fps, hdr:12(7), source, vcodec, channels, acodec, lang, proto, size, br, asr, vext, aext, hasaud, id
[info] Available formats for IM4zZchluX0:
ID  EXT   RESOLUTION FPS │ PROTO │ VCODEC MORE INFO
────────────────────────────────────────────────────
sb3 mhtml 48x27        0 │ mhtml │ images storyboard
sb2 mhtml 89x45        0 │ mhtml │ images storyboard
sb1 mhtml 179x90       0 │ mhtml │ images storyboard
sb0 mhtml 359x180      0 │ mhtml │ images storyboard

The new 2025.09.26 version fixes it.

Reproducible: Always