THE-7: Cross-browser MV3 parity fixes for Firefox and Chrome

[ConduciveMammal]

Summary

• Fix Firefox temporary add-on installation errors by emitting background scripts for Firefox builds.
• Keep background code classic-script compatible in Firefox while retaining cross-browser API guards.
• Add a shared extension API helper for popup and content contexts.
• Strip unsupported use_dynamic_url from Firefox manifests after build.
• Add cross-browser parity QA notes and update the release QA checklist.
• Update build verification to accept either MV3 service worker (Chrome) or background scripts (Firefox).

Verification

• npm run build:prod
• npm run build:firefox
• npm run qa:verify-build
• npm run package:bundle:firefox

[chatgpt-codex-connector]

💡 Codex Review

theme-explorer/src/pages/Popup/Popup.jsx

Lines 358 to 362 in 12330f3

	extensionApi.tabs.sendMessage(
	state.currentTab.id,
	{ popupIsOpen: true },
	(response) => {
	if (response && response.type === 'theme' && response.data) {

Avoid callback-style tabs.sendMessage on browser namespace

This call relies on a callback and runtime.lastError handling, but in Firefox getExtensionApi() prefers the browser namespace (src/utils/getExtensionApi.js), where tabs.sendMessage is promise-based and does not drive this callback path. That means the missing-receiver recovery branch (ensureContentScriptInjected/fallback) can be skipped on Firefox when no listener is present, so storefront detection can fail on tabs that are not already wired up.

---

theme-explorer/src/pages/Content/index.js

Lines 161 to 163 in 12330f3

	appendInjectScript('src/pages/Inject/index.js', (event) => {
	console.error('Error reloading script for popup request:', event);
	});

Prevent repeated inject script insertion during popup polling

When data is still empty, each popupIsOpen message appends a new inject script without checking whether one is already running. Since the popup polls this message repeatedly, a single popup open can create multiple injected instances, each with its own retry interval in src/pages/Inject/index.js, causing duplicated lookups/messages and unnecessary page-side work.

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