Firefox can block sites if they contain harmful content or if you’ve manually blocked them.
Some extensions block specific sites.
To check:
Common blocking extensions: uBlock Origin, AdBlock Plus, BlockTube, LeechBlock, parental control tools. how to unblock a site in firefox
Warning: Free HTTP proxies are insecure. Only use this for non-sensitive browsing like unblocking a news article. Firefox can block sites if they contain harmful