"What could be the problem?" Quite a few things, but they can be narrowed down fairly easily. What happens when you try to visit the page? Is it blank or does it give an error? Does it take time (is the loading animation on for a few seconds?) to fail, or does it feel instant? "I checked to see if the site is down but it's not" Were you on the same network? (behind same public IP/router/firewall) Did the site have an up to date certificate, with proper https? Were you on the same machine but using a different browser? If not, did you use Firefox? Most of the above is to rule out DNS, firewall, blacklisting, etc.. "I added the Firefox uBlock extension a few weeks ago to stop constant website popups (after I tried resetting Firefox) and I turned uBlock off for this certain website but I still can't access the website." This is my first response as a volunteer, so I am treading lightly here... This feels like a grey area that could be interpreted as[https://support.mozilla.org/en-US/kb/mozilla-support-rules-guidelines#w_subjects-covered-in-the-support-forum| "off-topic" because of the third party plug-in] listed. They do not always "disable" properly and even after a removal, don't all "clean up after themselves" "I also installed Mozilla VPN last week." I don't have experience with Mozilla's product/service. However, VPN issues tend to break a lot more than one website or "just work" on the user's (you/client) end. The website very well could have VPN blacklists in place. Ensure that your public IP is correct while troubleshooting this issue. Not knowing your environment, (Windows, Linux, Mac) but assuming you are able to connect from the machine, securely, with a separate browser. A sync, backup of downloads, uninstall, clearing system cache, OS update, reboot (not shutdown, sleep, hibernate, etc...) > Fresh install may be the cleanest and fastest solution. Please report back with the above information (for further assistance) or (hopefully) your steps to success!

Try changing [[Configure DNS over HTTPS protection levels in Firefox|DNS over HTTPS (DoH)]] to "Off" or "Max Protection" and in [[Connection settings in Firefox|Connection Settings]] change "Use system proxy settings" to "No proxy". Does it still happen in [[Diagnose Firefox issues using Troubleshoot Mode|Troubleshoot Mode]]?