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.

Each drain reads the console and clears it in ONE chrome script. Splitting
the read from Services.console.reset() left a window between the two round
trips in which an error was logged into a buffer about to be discarded, and
destroyed unread rather than deferred to the next drain; a probe of 100
sequenced throws at 20ms spacing lost one. With the drain atomic the same
probe accounts for every throw that falls inside the observed window, on two
consecutive runs.

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, with measurements rather
than absolutes. Error capture is poll-based, so an error is attributed to a
step and not to a moment within it; the drained window ends ~1.5s after the
last step returns (a 500ms settle, a 1000ms sleep and two drain round trips),
and that cut-off jitters run to run — three runs of throws at fixed offsets
reported everything up to +1.5s and one of the three also reported +1.6s.
Inside the window the atomic drain leaves no race, but nsIConsoleService
keeps a ring buffer of only 250 messages, so more than 250 console messages
between two drains evicts unread errors: 400 throws inside one step report
as exactly the newest 250, on three runs, while occupancy in a clean run
peaks at 4 of 250 at the install drain and 0 at every later drain. 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.

script/test-e2e-firefox

@@ -0,0 +1,63 @@
+#!/bin/sh
+# script/test-e2e-firefox: build the extension and drive the real popup in
+# a real Firefox inside a pinned container. The Firefox counterpart to
+# script/test-e2e. Our own extension to scripts-to-rule-them-all.
+#
+# Deliberately NOT called by script/check or script/test, for the same
+# reason as the Chrome suite: REPO_POLICIES.md caps make test at 20 seconds
+# and a browser suite does not fit.
+#
+# Unlike script/test-e2e this builds its image locally, because no
+# published image carries both a pinned Firefox and a matching geckodriver.
+# All three external artifacts are pinned by digest inside the Dockerfile;
+# see tests/e2e/firefox/Dockerfile.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+IMAGE="$("$SCRIPT_DIR/projectname")-e2e-firefox"
+
+main() {
+    cd "$ROOT"
+
+    if ! command -v docker >/dev/null 2>&1; then
+        echo "test-e2e-firefox: docker is required to run the e2e suite" >&2
+        exit 1
+    fi
+
+    echo "Building extension for e2e..."
+    yarn run build 2>&1
+
+    # The build context is tests/e2e/firefox/ and holds nothing but the
+    # Dockerfile: the harness itself arrives over the bind mount below, so
+    # editing it never invalidates an image layer.
+    echo "Building the pinned Firefox e2e image..."
+    docker build -t "$IMAGE" "$ROOT/tests/e2e/firefox"
+
+    echo "Running the Firefox e2e suite..."
+    # --shm-size=1g: Firefox needs more than the default 64MB /dev/shm.
+    # --network none: the suite stubs nothing, so this is what keeps the
+    #   run offline and deterministic. The extension swallows its own
+    #   fetch failures, so the popup flows work unchanged; see the
+    #   network note in README.md. Weaker than the Chrome suite's
+    #   fixture interception, and honestly so — it proves no request
+    #   escaped, but it cannot report which ones were attempted.
+    # --user: keep files the suite touches owned by the caller, not root.
+    # HOME=/tmp: the mapped uid has no home directory in the image.
+    #
+    # No --privileged. Firefox's sandbox logs
+    # "CanCreateUserNamespace() clone() failure: EPERM" on startup here;
+    # it is cosmetic and headless Firefox runs fine without it.
+    docker run --rm \
+        --shm-size=1g \
+        --network none \
+        --user "$(id -u):$(id -g)" \
+        -e HOME=/tmp \
+        -v "$ROOT:/work" \
+        -w /work \
+        "$IMAGE" \
+        node tests/e2e/firefox/run.js dist/firefox
+}
+
+main "$@"