Hello RexaFire.

When you browse the web, browsers store your browsing history (list of websites you have visited), what you have filled in the forms, cache images on the visited pages etc., to ease you filling forms in the future or make future page load faster if you have been on any of them before.

In some cases you may want to remove some of those data and this dialog is one of the options to do so. For more information, please read the Delete browsing, search and download history on Firefox article.

You probably recognize the first four choices

• Browsing and Download History
• Form and Search History, this is about autocomplete data for filling a form and for searches
• Cookies => Options/Preferences (search for cookies)
• Cache => this is the disk cache, see about:cache and Options/Preferences (search for cache)

• Active Logins is about accessing servers via basic authentication that require to fill a username and password dialog
• Offline Website Data, this is about a website storing data locally
  you can set browser.preferences.offlineGroup.enabled to true on the about:config page to show this section on Options/Preferences
• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an "Allow" exception you may want to keep

Deleting what stops notification from sites.

If you mean Web Push notifications in Firefox, you can remove the permissions for the sites in Firefox Options > Privacy & Security > Permissions section > Setting button next to the Notifications headline. In the coming release, or the next one, at the same place there will be an option to completely block sites from asking for the permissions.

There is also a website specific setting in "Tools -> Page Info -> Permissions -> Receive Notifications".

You can see all registered service workers on this page: about:serviceworkers

You can open "about:" pages via the location/address bar

• "about:" is a protocol to access special pages