diff --git a/core/ui_appearance.py b/core/ui_appearance.py
new file mode 100644
index 00000000..469029d2
--- /dev/null
+++ b/core/ui_appearance.py
@@ -0,0 +1,40 @@
+"""Pure decisions for UI-appearance defaults.
+
+Kept here (importable, no Flask/config coupling) so the rules are unit-testable in
+isolation; web_server does only the request/config plumbing around them.
+
+Worker orbs are a blurred 60fps canvas — the main remaining Firefox lag source after
+the #935 sweep. So for a FIRST-TIME user (no saved preference) we default them OFF on
+Firefox and ON everywhere else: a smooth first impression where it's needed, full
+polish where the browser handles it. An explicit saved choice ALWAYS wins — this only
+picks the default when the user hasn't chosen.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+
+def is_firefox_user_agent(user_agent: Optional[str]) -> bool:
+    """True when a User-Agent string is Firefox.
+
+    Used ONLY to pick a performance-friendly default — never to gate functionality —
+    so a spoofed or unusual UA simply gets the default and the user can toggle. Chrome,
+    Edge, Safari, Opera, Brave do not carry 'firefox' in their UA; Firefox does
+    ('… Gecko/… Firefox/<ver>')."""
+    return 'firefox' in str(user_agent or '').lower()
+
+
+def resolve_worker_orbs_default(explicit: object, is_firefox: bool) -> bool:
+    """Whether worker orbs should be on.
+
+    ``explicit`` is the saved config value: ``True``/``False`` when the user has chosen,
+    ``None`` when unset. An explicit choice always wins; when unset, default OFF on
+    Firefox (perf) and ON elsewhere.
+    """
+    if explicit is None:
+        return not is_firefox
+    return explicit is not False
+
+
+__all__ = ['is_firefox_user_agent', 'resolve_worker_orbs_default']
diff --git a/tests/test_ui_appearance.py b/tests/test_ui_appearance.py
new file mode 100644
index 00000000..1fd2f544
--- /dev/null
+++ b/tests/test_ui_appearance.py
@@ -0,0 +1,56 @@
+"""Pure UI-appearance default rules (core/ui_appearance.py).
+
+Pins the worker-orbs default contract: explicit saved choice ALWAYS wins; when unset,
+default OFF on Firefox (the blurred orb canvas is the main remaining Firefox lag
+source) and ON elsewhere."""
+
+from core.ui_appearance import is_firefox_user_agent, resolve_worker_orbs_default
+
+_FIREFOX_UA = ("Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:128.0) "
+               "Gecko/20100101 Firefox/128.0")
+_CHROME_UA = ("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 "
+              "(KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36")
+_SAFARI_UA = ("Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 "
+              "(KHTML, like Gecko) Version/17.0 Safari/605.1.15")
+_EDGE_UA = ("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 "
+            "(KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36 Edg/120.0.0.0")
+
+
+# ── is_firefox_user_agent ──
+
+def test_detects_firefox():
+    assert is_firefox_user_agent(_FIREFOX_UA) is True
+
+
+def test_non_firefox_browsers_are_false():
+    for ua in (_CHROME_UA, _SAFARI_UA, _EDGE_UA):
+        assert is_firefox_user_agent(ua) is False
+
+
+def test_empty_or_none_ua_is_not_firefox():
+    assert is_firefox_user_agent('') is False
+    assert is_firefox_user_agent(None) is False
+
+
+# ── resolve_worker_orbs_default: explicit ALWAYS wins ──
+
+def test_unset_defaults_off_on_firefox():
+    assert resolve_worker_orbs_default(None, is_firefox=True) is False
+
+
+def test_unset_defaults_on_elsewhere():
+    assert resolve_worker_orbs_default(None, is_firefox=False) is True
+
+
+def test_explicit_true_wins_even_on_firefox():
+    # A Firefox user who explicitly enabled orbs keeps them — default never overrides.
+    assert resolve_worker_orbs_default(True, is_firefox=True) is True
+
+
+def test_explicit_false_wins_even_off_firefox():
+    assert resolve_worker_orbs_default(False, is_firefox=False) is False
+
+
+def test_explicit_values_ignore_browser():
+    assert resolve_worker_orbs_default(True, is_firefox=False) is True
+    assert resolve_worker_orbs_default(False, is_firefox=True) is False
diff --git a/web_server.py b/web_server.py
index 2383b59f..c21a1fba 100644
--- a/web_server.py
+++ b/web_server.py
@@ -84,6 +84,7 @@ if not pp_logger.handlers:
     pp_logger.propagate = False
 from core.spotify_client import SpotifyClient, Playlist as SpotifyPlaylist, Track as SpotifyTrack, _is_globally_rate_limited as _spotify_rate_limited
 from core.plex_client import PlexClient
+from core.ui_appearance import is_firefox_user_agent, resolve_worker_orbs_default
 from plexapi.myplex import MyPlexAccount, MyPlexPinLogin
 from core.jellyfin_client import JellyfinClient
 from core.navidrome_client import NavidromeClient
@@ -359,12 +360,32 @@ def _hsl_to_rgb(hue, saturation, lightness):
     )
 
 
+def _request_is_firefox() -> bool:
+    """Whether the current request's browser is Firefox (UA-based). Used ONLY to pick a
+    performance-friendly default; an explicit saved setting always wins. Safe outside a
+    request context (returns False)."""
+    try:
+        from flask import has_request_context, request
+        if not has_request_context():
+            return False
+        return is_firefox_user_agent(request.headers.get('User-Agent'))
+    except Exception:
+        return False
+
+
 def _initial_appearance_context():
     preset = config_manager.get('ui_appearance.accent_preset', '#1db954')
     custom = config_manager.get('ui_appearance.accent_color', '#1db954')
     accent = _valid_hex_color(custom if preset == 'custom' else preset)
     particles_enabled = config_manager.get('ui_appearance.particles_enabled', False) is True
-    worker_orbs_enabled = config_manager.get('ui_appearance.worker_orbs_enabled', True) is not False
+    # Worker orbs: explicit choice wins; unset → OFF on Firefox (the blurred orb canvas
+    # is the main remaining Firefox lag source), ON elsewhere. config default None so we
+    # can tell "unset" from an explicit False. (#kettui — single source: the server
+    # decides, the client consumes the injected value below.)
+    worker_orbs_enabled = resolve_worker_orbs_default(
+        config_manager.get('ui_appearance.worker_orbs_enabled', None),
+        _request_is_firefox(),
+    )
     reduce_effects = config_manager.get('ui_appearance.reduce_effects', False) is True
     r, g, b = _hex_to_rgb(accent)
     hue, saturation, lightness = _rgb_to_hsl(r, g, b)
diff --git a/webui/static/settings.js b/webui/static/settings.js
index 36a5a803..69443963 100644
--- a/webui/static/settings.js
+++ b/webui/static/settings.js
@@ -1548,8 +1548,14 @@ async function loadSettingsData() {
         if (particlesCheckbox) particlesCheckbox.checked = particlesEnabled;
         applyParticlesSetting(particlesEnabled);
 
-        // Worker orbs toggle
-        const workerOrbsEnabled = settings.ui_appearance?.worker_orbs_enabled !== false; // default true
+        // Worker orbs toggle. When the user hasn't saved a preference, reflect the
+        // server-decided browser-aware default (window._workerOrbsEnabled — OFF on
+        // Firefox for perf) so saving settings doesn't silently flip a first-time
+        // Firefox user's orbs back on. An explicit saved config value always wins.
+        const _orbsCfg = settings.ui_appearance?.worker_orbs_enabled;
+        const workerOrbsEnabled = (_orbsCfg === undefined || _orbsCfg === null)
+            ? (window._workerOrbsEnabled !== false)
+            : (_orbsCfg !== false);
         const workerOrbsCheckbox = document.getElementById('worker-orbs-enabled');
         if (workerOrbsCheckbox) workerOrbsCheckbox.checked = workerOrbsEnabled;
         applyWorkerOrbsSetting(workerOrbsEnabled);