Usually, you will use Adblock Plus to block http and https resources. Those are two examples for an
URI scheme. If you read the Wikipedia entry, you will see that there are
a lot of other schemes, some of them also known for their use in the context of a web browser, like
ftp://ftp.mozilla.org ,
mailto:
foo@example.com ,
about:config ....
Firefox uses some of those schemes internally (specifically: "about", "chrome", "moz-safe-about", "moz-icon", "x-jsd" and probably "resource"). Adblock Plus would actually be able to block them and because it makes little sense to block crucial local browser components (like, for example, just about the whole user interface
) and to avoid any of those things getting blocked accidentally by a bad filter, it makes a lot of sense to whitelist those schemes.
Do the research yourself if you want to know anything more specific about the schemes, but here is a quick classification:
"cid", "mailbox", "imap", "pop" and "addbook" are related to Mail/Thunderbird
"news", "snews", "nntp" are related to Newsgroups/Usenet
"irc" is related to IRC (duh)
"javascript": I didn't check how blocking those would work or whether it works at all
"file" is used to load local files, removing this scheme would for example allow you to block local embedded images of a local webpage (which you could be developing). However, non-local websites cannot use "file" to access local files for security reasons, therefore blocking "file" uris would really only serve local filter testing purposes.
"data" is used to embed files directly into a document and might be the only scheme worth removing from the list as it is actually used by websites. You can then block embedded "data" files, but only after they have already been downloaded (they are part of the document itself, they have already been downloaded with it when Adblock Plus gets a chance to see them). Therefore you could probably just hide the item with the Element Hiding Helper without the hassle of removing the scheme from the whitelist.
Example page for "data" images:
en/chrome/
After removing the "data" scheme whitelist and restarting Firefox, you could use this to block the screenshot:
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|data:image/png;base64,iVBORw0K*7WGFjEZdiGhvPpSlMZXBrYygnNmVM5*TkSuQmCC|$domain=adblockplus.org
To instead simply hide it which pretty much has the same effect but without the requirement to tamper with whitelistschemes:
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adblockplus.org##img[src^="data:image/png;base64,iVBORw0K"][src*="7WGFjEZdiGhvPpSlMZXBrYygnNmVM5"][src$="TkSuQmCC"]