diff --git a/.travis.yml b/.travis.yml
index c9e346a..8c3a12d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,13 +1,16 @@
-sudo: true
-
 language: node_js
 node_js:
   - "lts/*"
 
+env:
+  - MOZ_HEADLESS=1
+
 # Setup headless Chrome support
 # https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-the-Chrome-addon-in-the-headless-mode
 addons:
   chrome: stable
+  firefox: latest
+
 before_install:
   - google-chrome-stable --headless --disable-gpu --remote-debugging-port=9222 http://localhost &
 
diff --git a/karma.conf.js b/karma.conf.js
index f6a8abd..b09f102 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -1,10 +1,8 @@
 module.exports = function(config) {
   config.set({
-    browsers: ['ChromeHeadless'],
     singleRun: true,
     basePath: '',
     files: ['tests/dist/index.js'],
-
     frameworks: ['mocha', 'chai'],
     reporters: ['mocha'],
     client: {
diff --git a/package-lock.json b/package-lock.json
index a643c21..58a7327 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -5227,6 +5227,12 @@
         "which": "^1.2.1"
       }
     },
+    "karma-firefox-launcher": {
+      "version": "1.1.0",
+      "resolved": "https://registry.npmjs.org/karma-firefox-launcher/-/karma-firefox-launcher-1.1.0.tgz",
+      "integrity": "sha512-LbZ5/XlIXLeQ3cqnCbYLn+rOVhuMIK9aZwlP6eOLGzWdo1UVp7t6CN3DP4SafiRLjexKwHeKHDm0c38Mtd3VxA==",
+      "dev": true
+    },
     "karma-mocha": {
       "version": "1.3.0",
       "resolved": "https://registry.npmjs.org/karma-mocha/-/karma-mocha-1.3.0.tgz",
diff --git a/package.json b/package.json
index bc3a1a9..e2da68d 100644
--- a/package.json
+++ b/package.json
@@ -26,8 +26,13 @@
     "test": "npm run karma-mocha",
     "prebuild": "parcel build --global Filer src/index.js --no-minify --out-file filer.js",
     "build": "parcel build --global Filer src/index.js --out-file filer.min.js --detailed-report",
-    "prekarma-mocha": "parcel build tests/index.js --no-source-maps --out-dir tests/dist",
-    "karma-mocha": "karma start karma.conf.js",
+    "build-tests": "parcel build tests/index.js --no-source-maps --out-dir tests/dist",
+    "prekarma-mocha-firefox": "npm run build-tests",
+    "karma-mocha-firefox": "karma start karma.conf.js --browsers FirefoxHeadless",
+    "prekarma-mocha-chrome": "npm run build-tests",
+    "karma-mocha-chrome": "karma start karma.conf.js --browsers ChromeHeadless",
+    "prekarma-mocha": "npm run build-tests",
+    "karma-mocha": "karma start karma.conf.js --browsers ChromeHeadless,FirefoxHeadless",
     "coverage": "nyc mocha tests/index.js"
   },
   "repository": {
@@ -45,6 +50,7 @@
     "karma": "^3.0.0",
     "karma-chai": "^0.1.0",
     "karma-chrome-launcher": "^2.2.0",
+    "karma-firefox-launcher": "^1.1.0",
     "karma-mocha": "^1.3.0",
     "karma-mocha-reporter": "^2.2.5",
     "mocha": "^5.2.0",
diff --git a/src/filesystem/implementation.js b/src/filesystem/implementation.js
index 7d53205..87b095d 100644
--- a/src/filesystem/implementation.js
+++ b/src/filesystem/implementation.js
@@ -37,7 +37,6 @@ var SuperNode = require('../super-node.js');
 var Node = require('../node.js');
 var Stats = require('../stats.js');
 var Buffer = require('../buffer.js');
-const { validateInteger } = require('../shared.js');
 
 /**
  * Update node times. Only passed times are modified (undefined times are ignored)
@@ -1269,12 +1268,6 @@ function truncate_file(context, path, length, callback) {
       if(!fileData) {
         return callback(new Errors.EIO('Expected Buffer'));
       }
-      try {
-        validateInteger(length, 'len');
-      }
-      catch (error) {
-        return callback(error);
-      }
       var data = new Buffer(length);
       data.fill(0);
       if(fileData) {
@@ -1842,7 +1835,7 @@ function write(fs, context, fd, buffer, offset, length, position, callback) {
   } else if(!ofd.flags.includes(O_WRITE)) {
     callback(new Errors.EBADF('descriptor does not permit writing'));
   } else if(buffer.length - offset < length) {
-    callback(new Errors.EIO('intput buffer is too small'));
+    callback(new Errors.EIO('input buffer is too small'));
   } else {
     write_data(context, ofd, buffer, offset, length, position, callback);
   }
@@ -1930,6 +1923,15 @@ function exists(fs, context, path, callback) {
   stat(fs, context, path, cb);
 }
 
+function validateInteger(value, callback) {
+  if (typeof value !== 'number') {
+    callback(new Errors.EINVAL('Expected integer', value));
+    return;
+  }
+
+  return value;
+}
+
 // Based on https://github.com/nodejs/node/blob/c700cc42da9cf73af9fec2098520a6c0a631d901/lib/internal/validators.js#L21
 var octalReg = /^[0-7]+$/;
 function isUint32(value) {
@@ -2380,6 +2382,8 @@ function truncate(fs, context, path, length, callback) {
   length = length || 0;
 
   if(!pathCheck(path, callback)) return;
+  if(validateInteger(length, callback) !== length) return;
+
   truncate_file(context, path, length, callback);
 }
 
@@ -2394,6 +2398,7 @@ function ftruncate(fs, context, fd, length, callback) {
   } else if(!ofd.flags.includes(O_WRITE)) {
     callback(new Errors.EBADF('descriptor does not permit writing'));
   } else {
+    if(validateInteger(length, callback) !== length) return;
     ftruncate_file(context, ofd, length, callback);
   }
 }
diff --git a/src/shared.js b/src/shared.js
index 4da76cc..5c3337b 100644
--- a/src/shared.js
+++ b/src/shared.js
@@ -1,5 +1,3 @@
-var Errors = require('./errors.js');
-
 function guid() {
   return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
     var r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
@@ -9,22 +7,7 @@ function guid() {
 
 function nop() {}
 
-function validateInteger(value, name) {
-  let err;
-
-  if (typeof value !== 'number')
-    err = new Errors.EINVAL(name, 'number', value);
-
-  if (err) {
-    Error.captureStackTrace(err, validateInteger);
-    throw err;
-  }
-
-  return value;
-}
-
 module.exports = {
   guid: guid,
-  nop: nop,
-  validateInteger: validateInteger,
+  nop: nop
 };