[popups] Fix mount transitions on Positioner in Firefox

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

Fixes #3419

It seems like Firefox needs an extra delay before changing the 'starting' transition state so the getDisabledMountTransitionStyles lingers for the correct amount of time.

Navigation Menu: https://deploy-preview-3424--base-ui.netlify.app/react/components/navigation-menu
Side animations also fixed: https://deploy-preview-3424--base-ui.netlify.app/experiments/anchor-side-animations

[pkg-pr-new]

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui-components/react@3424
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui-components/utils@3424
  

commit: 9090421

[mui-bot]

Bundle size report

ℹ️ Using snapshot from parent commit 1b23b4b (fallback from merge base c0a9981).

Bundle	Parsed size	Gzip size
@base-ui-components/react	▼-20B(0.00%)	▼-4B(0.00%)

Details of bundle changes

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[netlify]

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[mnajdova]

Work as expected 👌