test: containerized Firefox end-to-end harness (closes #184)

Drives the real popup in a real Firefox with dist/firefox/ installed as an
unpacked MV2 temporary add-on, via geckodriver. Covers popup load, wallet
creation through the UI, and the Add Token screen. Outside make check, like
the Chrome suite.

Zero npm dependencies: tests/e2e/firefox/driver.js is a WebDriver client
over global fetch and child_process against geckodriver's HTTP API. The
Dockerfile pins the node base image, the Firefox 153.0.3 tarball and
geckodriver 0.36.0 by digest.

Errors are read from the privileged nsIConsoleService in Marionette's chrome
context, filtered to non-warning entries whose sourceName is the extension
origin. BiDi log.entryAdded delivers nothing at all for extension pages, so
a Playwright-BiDi or Puppeteer-BiDi harness would see nothing and report
success; the code says so where someone would be tempted to simplify it.
Errors logged during add-on install and background startup are drained and
folded into step 1, never discarded: a throw at the top of
src/background/index.js kills the background page and fails the run.
Content-script capture is left as unverified, because --network none leaves
no http:// page for a content script to be injected into.

No driver layer is shared with the Chrome suite and the three UI steps are
written twice deliberately: the two backends have no common substrate, and
three steps do not pay for a shim.

Two limits are documented rather than papered over. Error capture is
poll-based, so an error is attributed to a step and not to a moment within
it, and an error logged more than ~1s after the last step is missed
entirely because the browser is torn down. Nothing is stubbed; the container
runs with --network none instead, which proves no request escaped, cannot
report which were attempted, and runs only the failure branches of
network-dependent code.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: bootstrap setup install test test-e2e lint fmt fmt-check check docker hooks build build-debug verify-build clean dev
+.PHONY: bootstrap setup install test test-e2e test-e2e-firefox lint fmt fmt-check check docker hooks build build-debug verify-build clean dev
 
 # Standard targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
@@ -16,10 +16,13 @@ install:
 test:
 	@script/test
 
-# Browser end-to-end suite. Requires docker; not part of check.
+# Browser end-to-end suites. Both require docker; neither is part of check.
 test-e2e:
 	@script/test-e2e
 
+test-e2e-firefox:
+	@script/test-e2e-firefox
+
 lint:
 	@script/lint