If you blocked the website in Adblock Plus then click the ABP button on the toolbar and check what content you blocked for this domain.

Open the Blockable Items pane, either by clicking the toolbar icon and choosing the respective menu option, or by pressing Ctrl+Shift+V.

You can inspect and manage the Firefox permissions for all domains on the about:permissions page.

• https://support.mozilla.org/kb/how-do-i-manage-website-permissions

You can inspect and manage the permissions for the domain in the currently selected tab via these steps:

• Click the "Site Identity Button" (globe/padlock) on the location bar
• Click "More Information" to open "Tools > Page Info" with the Security tab selected
• Go to the Permissions tab (Tools > Page Info > Permissions) to check the permissions for the domain in the currently selected tab

I take it you're referring to a filter in Adblock Plus, which you have installed.

1. Click the Adblock Plus icon on the toolbar and chose Open Blockable Items.
2. Go a page that has a filter you want to remove.
3. In the Blockable Items Pane, click the rightmost icon (below the search box, and to the right side of the Type column) and make sure Filter Source is checked in that menu.
4. In the list of blockable items, look for filters with the source "My Blocking Filters" or similar. Right-click (or Ctrl+click) a filter and choose Edit Filter in Effect.
5. In the filter preferences window that opens, clear the checkbox next to the filter to disable it, or press the Delete key to remove it.