doc: firefox_preferences works on linux and windows

[ilpianista]

Solves #23955

Checklist for submitter

• [x] Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes. See Changes files for more information.
• [ ] Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)
• [ ] Added support on fleet's osquery simulator cmd/osquery-perf for new osquery data ingestion features.
• [ ] Added/updated tests
• [ ] If paths of existing endpoints are modified without backwards compatibility, checked the frontend/CLI for any necessary changes
• [ ] If database migrations are included, checked table schema to confirm autoupdate
• For database migrations:
  • [ ] Checked schema for all modified table for columns that will auto-update timestamps during migration.
  • [ ] Confirmed that updating the timestamps is acceptable, and will not cause unwanted side effects.
  • [ ] Ensured the correct collation is explicitly set for character columns (COLLATE utf8mb4_unicode_ci).
• [ ] Manual QA for all new/changed functionality
• For Orbit and Fleet Desktop changes:
  • [ ] Orbit runs on macOS, Linux and Windows. Check if the orbit feature/bugfix should only apply to one platform (runtime.GOOS).
  • [ ] Manual QA must be performed in the three main OSs, macOS, Windows and Linux.
  • [ ] Auto-update manual QA, from released version of component to new version (see tools/tuf/test).