Host permissions are specified as match patterns, and each pattern identifies a group of URLs for which the extension is requesting extra privileges. For example, a host permission could be "*://developer.mozilla.org/*".

The extra privileges include:

• XMLHttpRequest and fetch access to those origins without cross-origin restrictions (even for requests made from content scripts)
• the ability to read tab-specific metadata without the "tabs" permission, such as the url, title, and favIconUrl properties of tabs.Tab objects
• the ability to inject scripts programmatically (using tabs.executeScript()) into pages served from those origins
• the ability to receive events from the webrequest API for these hosts
• the ability to access cookies for that host using the cookies API, as long as the "cookies" API permission is also included.
• bypassing tracking protection for extension pages where a host is specified as a full domain or with wildcards. Content scripts, however, can only bypass tracking protection for hosts specified with a full domain.

In Firefox, from version 56 onwards, extensions automatically get host permissions for their own origin, which is of the form:

moz-extension://60a20a9b-1ad4-af49-9b6c-c64c98c37920/

where 60a20a9b-1ad4-af49-9b6c-c64c98c37920 is the extension's internal ID. The extension can get this URL programmatically by calling extension.getURL():

browser.extension.getURL("");
// moz-extension://60a20a9b-1ad4-af49-9b6c-c64c98c37920/

API permissions are specified as keywords, and each keyword names a WebExtension API that the extension would like to use.

These permissions are available in Manifest V2 and above unless otherwise noted:

• activeTab
• alarms
• background
• bookmarks
• browserSettings
• browsingData
• captivePortal
• clipboardRead
• clipboardWrite
• contentSettings
• contextMenus
• contextualIdentities
• cookies
• debugger
• declarativeNetRequest
• declarativeNetRequestFeedback
• declarativeNetRequestWithHostAccess
• dns
• downloads
• downloads.open
• find
• geolocation
• history
• identity
• idle
• management
• menus
• menus.overrideContext
• nativeMessaging
• notifications
• pageCapture
• pkcs11
• privacy
• proxy
• scripting
• search
• sessions
• storage
• tabHide
• tabs
• theme
• topSites
• unlimitedStorage
• webNavigation
• webRequest
• webRequestBlocking
• webRequestFilterResponse
• webRequestFilterResponse.serviceWorkerScript

In most cases the permission just grants access to the API, with the following exceptions:

• tabs gives you access to privileged parts of the tabs API without the need for host permissions: Tab.url, Tab.title, and Tab.faviconUrl.
  • In Firefox 85 and earlier, you also need tabs if you want to include url in the queryInfo parameter to tabs.query(). The rest of the tabs API can be used without requesting any permission.
  • As of Firefox 86 and Chrome 50, matching host permissions can also be used instead of the "tabs" permission.
• webRequestBlocking enables you to use the "blocking" argument, so you can modify and cancel requests.
• downloads.open lets you use the downloads.open() API.
• tabHide lets you use the tabs.hide() API.

This permission is specified as "activeTab". If an extension has the activeTab permission, then when the user interacts with the extension, the extension is granted extra privileges for the active tab only.

"User interaction" includes:

• the user clicks the extension's browser action or page action
• the user selects its context menu item
• the user activates a keyboard shortcut defined by the extension

The extra privileges are:

• The ability to inject JavaScript or CSS into the tab programmatically, using browser.tabs.executeScript() and browser.tabs.insertCSS()
• Access to the privileged parts of the tabs API for the current tab: Tab.url, Tab.title, and Tab.faviconUrl.

The intention of this permission is to enable extensions to fulfill a common use case, without having to give them very powerful permissions. Many extensions want to "do something to the current page when the user asks".

For example, consider an extension that wants to run a script in the current page when the user clicks a browser action. If the activeTab permission did not exist, the extension would need to ask for the host permission <all_urls>. But this gives the extension more power than it needs: it could now execute scripts in any tab, any time it likes, instead of just the active tab and only in response to a user action.

Usually the tab that's granted activeTab is just the currently active tab, except in one case. The menus API enables an extension to create a menu item which is shown if the user context-clicks on a tab (that is, on the element in the tabstrip that enables the user to switch from one tab to another).

If the user clicks such an item, then the activeTab permission is granted for the tab the user clicked, even if it's not the currently active tab (as of Firefox 63, Firefox bug 1446956).

There are two permissions which enables the extension to interact with the clipboard:

clipboardWrite

Write to the clipboard using Clipboard.write(), Clipboard.writeText(), document.execCommand("copy") or document.execCommand("cut")

clipboardRead

Read from the clipboard using Clipboard.read(), Clipboard.readText() or document.execCommand("paste")

See Interact with the clipboard for more details.

The unlimitedStorage permission:

• Enables extensions to exceed any quota imposed by the storage.local API
• In Firefox, enables extensions to create a "persistent" IndexedDB database without the browser prompting the user for permission at the time the database is created.

 "permissions": ["*://developer.mozilla.org/*"]

In Manifest V2 only, request privileged access to pages under developer.mozilla.org.

  "permissions": ["tabs"]

Request access to the privileged pieces of the tabs API.

  "permissions": ["*://developer.mozilla.org/*", "tabs"]

In Manifest V2 only, request both of the above permissions.