User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:73.0) Gecko/20100101 Firefox/73.0

Steps to reproduce:

Web extensions can request permissions, including a special meta permisson named activeTab which allows being granted permissions only when the user invokes the extension.

Google's documentation for activeTab states that it allows to

intercept network requests in the tab to the tab's main frame origin using the webRequest API.
(https://developer.chrome.com/extensions/activeTab)

However, in mozilla's documentation about activeTab, there is no mention of the webRequest API.
(https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#activeTab_permission)

Also the incompatibility isn't mentioned in the chrome incompatibilities list.
(https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Chrome_incompatibilities)

Actual results:

An extension that uses activeTab to intercept network requests doesn't work in Firefox.

Expected results:

It would be very nice if it worked the same way as it does in Chrome. It would be nice for developers because extensions would be cross-browser compatible and require less permissions, and better for users because they wouldn't have to allow extensions that periodically need to intercept network requests to snoop on all of their network traffic all the time.

Adding this issue to the manifest v3 meta (so that we can re-evaluate if this would need to be bumped to an higher priority)

(changed from unconfirmed to new, because we already know that activeTab doesn't have currently any effect on what webRequest will be able to intercept, and so there is no need to require QA to try to verify this behavior explicitly).