Can I use
?

MPEG-4/H.264 video format

- OTHER

Commonly used video compression format.

Chrome

  1. ✅ 4 - 135: Supported
  2. ✅ 136: Supported
  3. ✅ 137 - 139: Supported

Edge

  1. ✅ 12 - 135: Supported
  2. ✅ 136: Supported

Safari

  1. ❌ 3.1: Not supported
  2. ✅ 3.2 - 18.4: Supported
  3. ✅ 18.5: Supported
  4. ✅ TP: Supported

Firefox

  1. ❌ 2 - 20: Not supported
  2. ◐ 21 - 34: Partial support
  3. ✅ 35 - 137: Supported
  4. ✅ 138: Supported
  5. ✅ 139 - 141: Supported

Opera

  1. ❌ 9 - 24: Not supported
  2. ✅ 25 - 116: Supported
  3. ✅ 117: Supported

IE

  1. ❌ 5.5 - 8: Not supported
  2. ✅ 9 - 10: Supported
  3. ✅ 11: Supported

Chrome for Android

  1. ✅ 136: Supported

Safari on iOS

  1. ✅ 3.2 - 18.4: Supported
  2. ✅ 18.5: Supported

Samsung Internet

  1. ✅ 4 - 26: Supported
  2. ✅ 27: Supported

Opera Mini

  1. ❌ all: Not supported

Opera Mobile

  1. ❌ 10: Not supported
  2. ✅ 11 - 12.1: Supported
  3. ✅ 80: Supported

UC Browser for Android

  1. ✅ 15.5: Supported

Android Browser

  1. ◐ 2.1 - 2.2: Partial support
  2. ◐ 2.3: Partial support
  3. ◐ 3 - 4.3: Partial support
  4. ✅ 4.4 - 4.4.4: Supported
  5. ✅ 136: Supported

Firefox for Android

  1. ✅ 137: Supported

QQ Browser

  1. ✅ 14.9: Supported

Baidu Browser

  1. ✅ 13.52: Supported

KaiOS Browser

  1. ✅ 2.5: Supported
  2. ✅ 3: Supported

Firefox supports H.264 on Windows 7 and later since version 21. Firefox supports H.264 on Linux since version 26 if the appropriate system libraries are installed.

Partial support for older Firefox versions refers to the lack of support in OS X & some non-Android Linux platforms.

Resources:
Firefox extension allowing support in Win7
Wikipedia article