My firefox can't search some sites & vpn extentions don't work correctly.

Hello. I'm iranian and i need to use vpn to search some sites like pinterest, youtube, freepik&... . for that i add veepn extention on my firefox. It connects but doesn't run on the site also till using the extention, browsing gets some problems and my browser doesn't work properly. Even recently it works slower. I don't know what is the problem to fix it; so I tried to ask your team. Thanks for your attention.