A bunch of nit-picking grammar fixes fixes and suggestions for rephrasing:

Before:
This feature allows to automatically whitelist temporarily the whole domain of a page whenever it is first loaded in the browser.

After:
This feature enables automatic temporary whitelisting of a page's whole domain whenever it is first loaded in the browser.

Before:
Although security-wise it is not a recommanded feature to use

After:
Although it is not a recommended feature from a security perspective

Before:
I rather give users this feature

After:
I would rather give users this feature

Before:
Delete blocked cookies

After:
Delete blocked cookies.

Before:
Delete non-blocked session cookies (x) minutes after last time they have been used.

After:
Delete non-blocked session cookies (x) minutes after the last time they have been used.

Before:
Delete contents of local storage of blocked hostnames

After:
Delete local storage content set by blocked hostnames.

Before:
Remove third-party HTTP referer information for non-whitelisted hostname.

After:
Remove third-party HTTP referer information from non-whitelisted hostnames.

Before:
To allow/block in whole or part behind-the-scene requests you must now use the matrix which is available when clicking the extension icon while on this page.

After:
You must now use the matrix to allow or block behind-the-scene requests. Click the extension icon while viewing this page to get started.

Before:
HTTP Switchboard let you intercept and treat

After:
HTTP Switchboard lets you intercept and treat

Before:
Potentially blocking behind-the-scene net requests is

After:
Blocking behind-the-scene net requests is potentially

Before:
Even if this feature is not enabled, behind-the-scene requests are still logged by HTTP Switchboard, so that you can at least inspect them (from the Stats page).

After:
Even if this feature is not enabled, behind-the-scene requests are still logged by HTTP Switchboard. You can inspect these requests on the Stats page.