Add the Firefox tests back #263

The 1.0.0-rc.2 release gate failed in Firefox (run 38763042004): all four variants of #261's "a pinned bar dragged into the hours keeps its title at its copy's start" (tools/a11y/tests/scrolling-axis-lock.a11y.tsx) timed out waiting for the drag copy. Chromium and WebKit pass it. Firefox runs only in CI's on-demand jobs and the release gate: Playwright's Firefox doesn't start on macOS 27 (#182), and CI's PR jobs run Chromium alone. So nothing caught it before the release.

To unblock the release candidate, vp run browsers leaves Firefox out until this issue brings it back. That covers the full gate, the release gate and CI's on-demand job. Nothing is decided yet about why the copy doesn't appear in Firefox: it may be the test's synthetic drag, as WebKit's was, or a real bug in dragging an all-day bar out of the header.

Done when

• The cause of the Firefox failure is known, and the code or the test is fixed.
• vp run browsers runs Firefox again, and the full suite passes there in CI's on-demand job.
• The docs that list Firefox as left out (AGENTS.md, docs/internal/quality-gates.md) say it runs again.