While in Firefox, press the Alt key on your keyboard to display the main menu bar. Now, go to Tools and click on Website Blocker. A new window will open up where you will see all the websites that are currently blocked and for how long they're blocked. Try removing the website that you want to access from that list.
In case that doesn't work, you may need to disable and/or remove the extension. If you don't want to do that either, try instructing the extension to only block the website for a limited amount of time and set that period to one more day.
As a personal opinion, I would uninstall Website Blocker as soon as I could. The extension wants you to "upgrade for free to the Pro version" in order to actually do its job, which is already very fishy. I recommend trying BlockSite or LeechBlock instead.