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Firefox 149 adds built-in free VPN with 50GB monthly data
(www.omgubuntu.co.uk)
This is a most excellent place for technology news and articles.
When you connect to a secure https site, then no. When you connect to unsecured http, then yes.
Firefox has its own cert store for https, and could theoretically impersonste other websites by adding their own and then sniff your traffic. But thats not how vpn providers typically work, and thered be huge backlash if they were to do that unprompted.