If you click the menu button (the three lines in the top-right of the screen), you can select "Add-ons and themes". This will open a new tab that shows all add-ons that are currently installed on Firefox. You can enable or disable any of these add-ons using the toggle switch button (to the left of the button with three dots). This will disable the add-on on all websites. However, many adblock add-ons offer a whitelisting feature to disable only on specific websites. Setting that up will depend on the adblock that you are using. There are usually instructions on how to do that on your adblock's support website. Hope this helps.