In addition to removing black bars from videos, Ultrawidify offers advanced options to fix the aspect ratios.

It offers you several options to help you get rid of the black bars:

If the video does not utilize DRM, you can just sit back and let autodetection handle everything for you. On sites that use DRM — or on videos where autodetection proves unreliable — you can either use the extension popup or the keyboard shortcuts (note that extension shortcuts do not override the site's default shortcuts) to set the aspect ratio manually.

In addition, if the site doesn't support autodetection or if the autodetection is unreliable, you can manually crop the video to a specific aspect ratio.