fix(firefox): do not override Accept-Language header set by the page

[yury-s]

Summary

• Do not merge the locale-derived Accept-Language header into Firefox extra HTTP headers — the browser-side language override already produces the header, and the extra-header mechanism was clobbering an Accept-Language explicitly set on a fetch/XHR request.
• Add a test that an explicit Accept-Language on a fetch is preserved and that the locale-derived one is still sent when the request does not specify it. Marked fixme on WebKit until the corresponding browser fix rolls.

References #23732 (fixes the Firefox part; WebKit needs a separate fix).

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [chromium-library] › library/beforeunload.spec.ts:130 › should support dismissing the dialog multiple times `@chromium-ubuntu-22.04-node24`
⚠️ [firefox-page] › page/page-event-request.spec.ts:181 › should return response body when Cross-Origin-Opener-Policy is set `@firefox-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:388 › should reveal errors in the sourcetab `@windows-latest-node22`

49973 passed, 1189 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

2 failed
❌ [webkit] › mcp/annotate.spec.ts:110 › should abort annotation when last screenshot is removed @mcp-macos-latest-webkit
❌ [webkit] › mcp/annotate.spec.ts:173 › user-initiated annotate downloads zip with feedback.md @mcp-macos-latest-webkit

7758 passed, 1249 skipped

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Merge workflow run.

[github-actions]

Hi, I'm the Playwright bot and I took a first look at the CI failures.

🟢 The failures are pre-existing flakes — this PR is clear

The only two failures are mcp/annotate.spec.ts on WebKit, and this PR touches Firefox ffBrowser.ts plus a locale test — nothing in the MCP annotate path. Both tests flip verdict across runs on SHAs this PR can't be responsible for.

Details

Pre-existing flake / infra

• [webkit] › mcp/annotate.spec.ts:110 › should abort annotation when last screenshot is removed — flake. Across the test-results DB this WebKit test failed 4 of 316 runs and passed the other 312, including a failure on push sha 97a80eb8 and on unrelated PR chore(recorder): move signals from the action onto the action context #41755 — none of them this PR. It also flakes on chromium/chrome/firefox, so it isn't engine-specific.
• [webkit] › mcp/annotate.spec.ts:173 › user-initiated annotate downloads zip with feedback.md — flake. Failed 11 of 316 WebKit runs, passing the rest, on unrelated PRs devops: report CI test results as parquet artifacts #41669, devops(test-results-db): CLI to compact CI test results into DuckDB #41673, fix(reporter): expose setup/teardown projects to preprocessSuite #41731, fix(aria): do not hang ai snapshot on frameset pages #41816 and several push SHAs (84d2d004, 696225c6, 449349ca, …). Same bimodal pattern on firefox (14/309) and chrome (3/315).

The PR's diff is confined to the Firefox Accept-Language header logic and browsercontext-locale.spec.ts; the MCP annotate flow exercises neither. The tests 1 report had only 3 flaky (no failures).

_{Triaged by the Playwright bot - agent run}