That's actually easier if you click the 'Edit filters as raw text' at the bottom of the 'Add your own filters' tab. This will automatically change the filter you've added into something like if you are on Chrome or Opera, there is currently no way to do that automatically - you will have to go into Adblock Plus options and edit the filter manually.

Click the cross symbol in the upper-right corner of the blockable items list or press Ctrl+Shift+V again to close blockable items.

Choose 'Disable this filter on ' where is the site you are on.There will be only one entry, right-click it.Choose 'Open blockable items' from the menu (you can also press Ctrl+Shift+V).When on the website, click the Adblock Plus icon.Currently that's significantly simpler in Firefox: The tricky part is updating the filter whenever you want to allow blocking on a particular website. Enter the filter into the text field and click 'Add filter'.Click 'Add filter' button on the right and enter this filter.If you don't have any filter group yet, click 'Add filter group'.Choose 'Filter Preferences' from the menu (you can also press Ctrl+Shift+F).First of all you need to add the custom filter that will have the effect of disabling Adblock Plus everywhere. It is somewhat awkward however, the user interface support for this feature is still very lacking. Yes, running Adblock Plus in a whitelist mode is possible.

I cannot answer your question for other extensions, some of them implemented the Adblock Plus filter syntax in a slightly broken way. I am assuming that you are asking about Adblock Plus.

Adblock plus firefox extension