diff --git a/src/auth/firefox.ts b/src/auth/firefox.ts
index d40e6ab..40b3d36 100644
--- a/src/auth/firefox.ts
+++ b/src/auth/firefox.ts
@@ -1,5 +1,6 @@
 import { existsSync } from "node:fs";
 import { join } from "node:path";
+import { snappyUncompress } from "hysnappy";
 
 import {
   copySqliteForRead,
@@ -19,6 +20,35 @@ export type FirefoxExtracted = {
 // eslint-disable-next-line no-control-regex
 const CONTROL_CHAR_RE = /[\u0000-\u001F]/g;
 
+function decodeStorageValue(value: unknown, compressionType: number): unknown {
+  if (compressionType === 0) {
+    return value;
+  }
+  if (compressionType !== 1 || !(value instanceof Uint8Array)) {
+    return null;
+  }
+
+  // Snappy starts with the uncompressed byte length as a uint32 varint.
+  let length = 0;
+  for (let i = 0; i < Math.min(value.length, 5); i++) {
+    const byte = value[i]!;
+    length += (byte & 0x7f) * 2 ** (7 * i);
+    if (byte & 0x80) {
+      continue;
+    }
+    // Bound allocations from corrupt profile data before invoking the decoder.
+    if (length > 64 * 1024 * 1024) {
+      return null;
+    }
+    try {
+      return snappyUncompress(value, length);
+    } catch {
+      return null;
+    }
+  }
+  return null;
+}
+
 function toStringValue(value: unknown): string {
   if (typeof value === "string") {
     return value;
@@ -167,11 +197,15 @@ async function extractTeamsFromProfile(
     try {
       const rows = (await queryReadonlySqlite(
         copied.copyPath,
-        "select key, value from data where key in ('localConfig_v2', 'localConfig_v3') order by key desc",
-      )) as { key: string; value: unknown }[];
+        "select key, value, compression_type from data where key in ('localConfig_v2', 'localConfig_v3') order by key desc",
+      )) as { key: string; value: unknown; compression_type: number }[];
 
       for (const row of rows) {
-        const cfg = parseJsonObjectFromValue(row.value);
+        const value = decodeStorageValue(row.value, row.compression_type);
+        if (value === null) {
+          continue;
+        }
+        const cfg = parseJsonObjectFromValue(value);
         const teamsRaw =
           cfg && typeof cfg.teams === "object" && cfg.teams !== null ? cfg.teams : {};
         const parsedTeams = Object.values(teamsRaw)
@@ -181,7 +215,7 @@ async function extractTeamsFromProfile(
           return { teams: parsedTeams, sourcePath: dbPath };
         }
 
-        const rawTeams = extractTeamsFromRawText(toStringValue(row.value));
+        const rawTeams = extractTeamsFromRawText(toStringValue(value));
         if (rawTeams.length > 0) {
           return { teams: rawTeams, sourcePath: dbPath };
         }
diff --git a/test/firefox-auth.test.ts b/test/firefox-auth.test.ts
new file mode 100644
index 0000000..7dd86d0
--- /dev/null
+++ b/test/firefox-auth.test.ts
@@ -0,0 +1,88 @@
+import { describe, expect, test } from "bun:test";
+import { Database } from "bun:sqlite";
+import { mkdtemp, mkdir, rm, writeFile } from "node:fs/promises";
+import { tmpdir, platform } from "node:os";
+import { join } from "node:path";
+import { snappyCompress } from "hysnappy";
+
+const team = { name: "Example", url: "https://example.slack.com/", token: "xoxc-test-token" };
+const config = JSON.stringify({ teams: { T123: team }, padding: "repeat ".repeat(100) });
+
+async function extractFixture(value: string | Uint8Array, compressionType: number) {
+  const home = await mkdtemp(join(tmpdir(), "agent-slack-firefox-test-"));
+  try {
+    const base =
+      platform() === "darwin"
+        ? join(home, "Library", "Application Support", "Firefox")
+        : join(home, ".mozilla", "firefox");
+    const profile = join(base, "fixture");
+    const storage = join(profile, "storage", "default", "https+++app.slack.com", "ls");
+    await mkdir(storage, { recursive: true });
+    await writeFile(
+      join(base, "profiles.ini"),
+      "[Profile0]\nName=fixture\nIsRelative=1\nPath=fixture\nDefault=1\n",
+    );
+    // Real SQLite files exercise discovery, snapshotting, querying and decoding together.
+    const db = new Database(join(storage, "data.sqlite"));
+    db.run("CREATE TABLE data (key TEXT, value BLOB, compression_type INTEGER)");
+    db.run("INSERT INTO data VALUES (?, ?, ?)", ["localConfig_v2", value, compressionType]);
+    db.close();
+    const cookies = new Database(join(profile, "cookies.sqlite"));
+    cookies.run("CREATE TABLE moz_cookies (host TEXT, name TEXT, value TEXT)");
+    cookies.run("INSERT INTO moz_cookies VALUES ('.slack.com', 'd', 'xoxd-test-cookie')");
+    cookies.close();
+    // Avoid changing HOME or mocking shared modules in the test runner.
+    const proc = Bun.spawn(
+      [
+        process.execPath,
+        "-e",
+        `
+      import { extractFromFirefox } from "./src/auth/firefox.ts";
+      const result = await extractFromFirefox();
+      console.log(JSON.stringify(result && { teams: result.teams, cookie_d: result.cookie_d }));
+    `,
+      ],
+      { env: { ...process.env, HOME: home }, stdout: "pipe", stderr: "pipe" },
+    );
+    const output = await new Response(proc.stdout).text();
+    const error = await new Response(proc.stderr).text();
+    expect(await proc.exited, error).toBe(0);
+    return JSON.parse(output);
+  } finally {
+    await rm(home, { recursive: true, force: true });
+  }
+}
+
+describe("Firefox auth storage", () => {
+  for (const [name, value, compression] of [
+    ["uncompressed text", config, 0],
+    ["uncompressed blob", Buffer.from(config), 0],
+    ["Snappy-compressed blob", snappyCompress(Buffer.from(config)), 1],
+  ] as const) {
+    test(`extracts ${name}`, async () => {
+      expect(await extractFixture(value, compression)).toEqual({
+        teams: [team],
+        cookie_d: "xoxd-test-cookie",
+      });
+    });
+  }
+
+  test("does not parse corrupt compressed data as plaintext", async () => {
+    expect(await extractFixture(Buffer.from(config), 1)).toBeNull();
+  });
+
+  for (const [name, value] of [
+    ["empty", Buffer.alloc(0)],
+    ["truncated length", Buffer.from([0x80])],
+    ["unterminated length", Buffer.from([0x80, 0x80, 0x80, 0x80, 0x80])],
+    ["oversized allocation", Buffer.from([0xff, 0xff, 0xff, 0xff, 0x0f])],
+  ] as const) {
+    test(`ignores ${name} compressed data`, async () => {
+      expect(await extractFixture(value, 1)).toBeNull();
+    });
+  }
+
+  test("ignores unsupported compression types", async () => {
+    expect(await extractFixture(Buffer.from(config), 2)).toBeNull();
+  });
+});