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I think this PR is not needed. I mean, if it is available in Nightly without a flag, then just setting "preview" is all we need.

The flag however is needed for Beta & Developer…

I see. Well, this structure would do the trick:

"firefox": [
  {
    "version_added": "preview"
  },
  {
    "version_added": "140",
    "flags": [
      {
        "type": "preference",
        "name": "dom.viewTransitions.enabled",
        "value_to_set": "true"
      }
    ]
  }
],

Hi @dletorey 👋🏼

As far as I can tell, Firefox Nightly currently (tested for 143 and 144) only supports Level 1 of the View Transition API plus view-transition-class and view-transition-name: match-element, but not the options object for document.startViewTransition() (see #27594).

Edit: TIL :active-view-transition is also supported, thank you for pointing out!