Sadly there is more of this going around today, web sites putting up the shields against hackers, bots, AI systems scraping their content, etc. to the point that legitimate users sometimes get locked out. Having said that, I have not been able to duplicate this issue with www.usps.com (is that the exact URL you're using?) or other variations, and I have non-Firefox web browsers here that many sites really don't like.

It's a bit of a cliché, but start by clearing the cache, cookies, restarting the computer, etc. Also try Troubleshooting Mode (Help -> Troubleshooting Mode) to run without addons.

What I'm most curious about is anything unusual about your internet connection. Are you located within the United States or North America? Are you using a VPN? I expect USPS is meant to be very international, and I'm honestly surprised you're having this problem at all on a non-military government site, but situations like this in my experience are near the top of things that can trip you up.

"You don't have permission to access..." errors are characteristic of a firewall that protects the server against misuse and this firewall thinks something is suspicious about the HTTP request. If you use a proxy or VPN, try to use a direct connection. Select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

• Settings -> General -> Network -> Connection -> Settings

You can try to clear the Cache and remove the Cookies for websites that cause problems via the padlock button or via "3-bar" Firefox menu button (Settings) to see if that fixes it.

• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

I've seen cases where having enabled Resist Fingerprinting was causing this issue.

• https://support.mozilla.org/en-US/kb/firefox-protection-against-fingerprinting

Start Firefox in Troubleshoot Mode to check if an extension is causing the problem.

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

If this all doesn't work, try "Forget About This Site" in the right-click context menu of an history item. Note that this removes saved passwords for this website as well.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox