Firefox performance optimization (Fedora / Linux)

Goal: make Firefox fast and stable — "no glitches, but holds up". Sources: Betterfox

(yokoffing/Betterfox, actively maintained, 10.7k stars, last check 2026-08-07,

Fastfox.js version 152) + general performance guide (SysAdminSage 2026-06).

Profile location GOTCHA (this box, updated 2026-08-10)

Firefox was Flatpak → migrated to RPM; profiles now live in

~/.mozilla/firefox/ (system Firefox, NOT flatpak sandbox dirs):

• 9ud3litf.default-release = ACTIVE profile (migrated from Flatpak 2026-08-10).

• j3z6402p.default-release = old system profile, preserved.

• profiles.ini marks 9ud3litf as Default; [Install...] Default= = active.

• Flatpak profiles lived in ~/.var/app/org.mozilla.firefox/config/mozilla/firefox/

  (XDG_CONFIG_HOME override) — if a "missing" profile hunt starts, check there

  first when the process runs from /app/ via bwrap (flatpak).

• If no profile dir exists at all: Firefox was never launched — run

  firefox -CreateProfile default or just start Firefox once.

SLOW VIDEO — diagnosis method & treatment (case 2026-08-10)

Symptom: streams/movies stutter, "everything is slow, it's the browser".

How to find the real cause (do NOT guess — measure, in this order):

1. Is it even a system package? pgrep -af firefox → if paths start with

   /app/ and bwrap is in the cmdline → it's Flatpak (this matters, see GOTCHA).

2. Does the GPU work at all? nvidia-smi while playing a video:

   near-0% GPU util + ~11W idle → video is NOT decoded by GPU.

3. Which codec path? grep vaapi ~/.mozilla/firefox/*/prefs.js — empty means

   software decode (CPU chokes on 1080p60, i5-9400F has NO iGPU — 'F' suffix).

4. Is the VA-API driver installed? vainfo — missing nvidia_drv_video.so

   and "VA-API NVDEC driver" line → hardware decode unavailable.

5. Research first (not memory): README elFarto/nvidia-vaapi-driver (Firefox

   Integration section), Fedora wiki Hardware_Video_Acceleration,

   discourse.flathub.org. Details saved in research.db (findings id=65, id=69).

Treatment (NVIDIA + Fedora, verified)

• System Firefox: sudo dnf install firefox (RPM — canonical for Fedora).

• nvidia-vaapi-driver is NOT packaged for Fedora (pkgs.org verified) — build:

  git clone https://github.com/elFarto/nvidia-vaapi-driver →

  sudo dnf install meson libva-devel gstreamer1-plugins-bad-freeworld libdrm-devel

  → meson setup build && sudo meson install -C build

  (ffnvcodec-headers pulled via meson wrap automatically).

• user.js in profile (Firefox 137+ requires force-enabled):

  | Pref | Value | Why |

  |---|---|---|

  | media.ffmpeg.vaapi.enabled | true | VA-API on |

  | media.hardware-video-decoding.force-enabled | true | REQUIRED since FF137 |

  | media.rdd-ffmpeg.enabled | true | decode in RDD process |

  | widget.dmabuf.force-enabled | true | DMA-BUF for NVIDIA |

  | media.av1.enabled | false | 1660 SUPER has no AV1 |

• Environment (system-wide, /etc/environment):

  MOZ_DISABLE_RDD_SANDBOX=1, LIBVA_DRIVER_NAME=nvidia, NVD_BACKEND=direct

  (EGL backend is broken on NVIDIA drivers 525+).

• Verify: vainfo shows VA-API NVDEC driver [direct backend] with

  H264/HEVC/VP8 profiles; nvidia-smi during playback shows non-zero decode.

• 1660 SUPER has NO VP9/AV1 NVDEC (vainfo list) → YouTube will still

  software-decode VP9 → install your-codecs extension (see firefox-extensions

  skill) to force H.264.

How to apply tweaks (user.js)

1. Firefox MUST be closed (user.js is read at startup; it overrides prefs every start).

2. Write user.js into the profile root:

   ~/.config/mozilla/firefox/<profile>/user.js

3. Values are applied on next launch; about:config shows them (search the pref name).

4. Rollback: delete user.js + restart.

Applied set (2026-08-07, machine: 16 GB RAM, GTX 1660 SUPER, Fedora 44)

Verified, non-breaking tweaks from Betterfox Fastfox.js (uncommented section):

| Pref | Value | Why |

|---|---|---|

| gfx.content.skia-font-cache-size | 20 | font render cache (MB), default 5 |

| gfx.canvas.accelerated.cache-size | 512 | GPU Canvas2D cache, default 256 |

| content.notify.interval | 100000 | page reflow timer (.10s), default 120000 |

| javascript.options.baselinejit.threshold | 50 | fewer dropped frames, default 100 |

| network.buffer.cache.size | 65535 | bigger packets, default 32768 |

| network.buffer.cache.count | 48 | default 24 |

| network.http.max-connections | 1800 | default 900 |

| network.http.max-persistent-connections-per-server | 10 | default 6 |

| network.http.max-urgent-start-excessive-connections-per-host | 5 | default 3 |

| network.http.request.max-start-delay | 5 | default 10 |

| network.dnsCacheExpiration | 3600 | DNS cache 1h, default 60 |

| media.cache_readahead_limit | 3600 | video buffer 10 min, default 60 |

| media.cache_resume_threshold | 1800 | video resume 5 min, default 30 |

| image.mem.decode_bytes_at_a_time | 32768 | default 16384 |

| browser.tabs.min_inactive_duration_before_unload | 300000 | unload inactive tabs after 5 min, default 600000 |

| browser.low_commit_space_threshold_mb | 13107 | 4/5 of 16 GB RAM (4GB=3276, 8GB=6553, 16GB=13107, 32GB=25698) |

| dom.ipc.processPrelaunch.fission.number | 1 | fewer preallocated processes, default 3 |

Full current Fastfox.js reference:

https://raw.githubusercontent.com/yokoffing/Betterfox/main/Fastfox.js

(some entries there are commented out = DEFAULTS, do not enable blindly).

GUI settings (do once, manual)

• Settings → Performance → uncheck "Use recommended performance settings" →

  keep "Use hardware acceleration when available" CHECKED (NVIDIA proprietary

  driver + Wayland: WebRender works, no need for software fallback).

• Install uBlock Origin (biggest single speedup — blocks heavy ads/trackers).

• Check rendering: about:support → "Compositing" line must be WebRender

  (NOT "Software WebRender" — that means GPU accel is broken → check driver,

  sudo dnf install akmod-nvidia, see fedora-warm-colors skill §GPU).

Do NOT touch (breaks stability)

• gfx.webrender.software / gfx.webrender.all — forces CPU rendering

• fission.* (webContentIsolationStrategy) — process isolation, Fission is default

• network.http.pacing.requests.enabled — network pacing, marginal gain

• browser.cache.memory.capacity — auto (32 MB) is fine for 16 GB machines

• dom.ipc.processCount* — process count is auto-managed since Fission

Verify

• about:support → Compositing: WebRender; GPU #1: NVIDIA... (or software — bad)

• about:processes → resource usage per tab

• about:config → search any pref from the table above, value must match