There isn't a global list, unfortunately. Instead, while you are on the site you want to revoke for, you can try either of these methods:

(1) Plugin notification icon

The plugin notification icon in the address bar typically looks like a small, dark gray Lego block to the left of the address. If you click it, does it let you undo your earlier decision?

(2) Permissions panel of the Page Info dialog

While on the site, you can call that up using any of these:

• right-click a blank area of the page and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions
• click the padlock or "i" icon to the left of the site address, then the ">" icon, then More Information > Permissions

Near the top, you'll find plugin permission, and you can change the setting for Adobe Flash for the site. (There's no save button, Firefox should save the change as soon as you make it.)

This extension can list out plugin permissions and I think you can delete them from its dialog:

https://addons.mozilla.org/firefox/addon/click-to-play-manager/

Last time I looked at it, I thought there was some important information missing, but I can't recall at this point.

Firefox would normally show all exceptions you make on a website in the control center 'i' button drop-down list.

• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

Such a choice is stored as part of the Site Preferences in content-prefs.sqlite.

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History").

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks and history and cookies and passwords and cache and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

Can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.