This is a quick rundown of all permissions required for this extension. Permissions below are named the way Firefox names them. Names of the permissions may be different in other browsers.

• Access (and modify) your data for all websites — in order to detect whether a page has video and whether said video needs to be corrected, this extension needs to inject a script into every webpage you visit. This — as well as adding extra CSS classes to <video> element if incorrect aspect ratio is detected — counts as "modifying the page," which necessitates this permission.

• Access browser tabs — is needed for controlling the extension from the popup. This permission is also needed for injecting custom CSS into the webpage, which is how this extension actually fixes the video.
  Since some websites had this nasty habit of fighting Ultrawidify when fixing aspect ratio via style attribute (which resulted in mad twitching), a different approach was needed. Long story short: "user styles" will always trump the CSS defined by a webpage, so Ultrawidify uses that to change video size. In order to add user styles to a webpage, you need this permission.
  Some sites also require us to define and inject some extra CSS that overrides what we get — for example: on vimeo, additional CSS is needed in order to force its player to take all the available width. (If the player doesn't take all the available width, cropping video to fit the screen is not possible.)

• Access browser activity during navigation — this permission allows extension to detect when you switch tabs (and which tab you're currently using). When you change tabs, extension pauses automatic detection in the tab you left and enables it in the tab you switched to (assuming the tabs have videos in them). This is a good idea for obvious reasons.

You will be asked to grant these permission when there's a need.

• Download files and read and modify the browser’s download history — required for exporting settings and saving logs.