Trim: firefox launcher, debugger-arm migration, comments and docs

- firefox: the prefs.js patterns built once; the extension link condition
  without the redundant desktopFix (it implies a pref).
- install.sh steamvr-debugger-arm: no second copy of the old-marker
  migration (cleanup, run on every switch before the arm unit exists,
  already does it).
- steam-ui-patches: shorter state option description; lib's description
  mentions extraArgs.
- README: uiPatches.patches fields in the options table (moved from
  docs/ui-patches.md); docs/ui-patches.md: the finder helpers;
  docs/steamvr-debugger.md: repeated sentences removed.

docs/steamvr-debugger.md

@@ -44,12 +44,6 @@ Port: `VRWebHelper/DebuggerPort`, default 8087.
 - when SteamVR stops, the key goes back to its previous value (removed if
   it wasn't there) and `.armed` is removed.
 
-The runtime files don't need Nix, which is why rollback and uninstall are
-covered; they are gone at reboot, and `.armed` (the only trace after a
-power loss) is resolved by the next SteamVR start or
-`steam-frame-nix-cleanup`. Disabled, there is no unit; the switch (cleanup)
-restores the key if SteamVR is stopped, otherwise the runtime drop-in does
-when it stops.
-
-Until SteamVR has been restarted once with the drop-in, the port is closed
-and `steam-ui-patches` keeps polling it.
+The runtime files need no Nix (rollback, uninstall) and are gone at reboot;
+`.armed`, the only trace after a power loss, is resolved by the next
+SteamVR start or `steam-frame-nix-cleanup`.