FoxyProxy has many DNS leaking bugs associated with the extension, especially when using SOCKS5 proxies. That extension has not been a recommended choice for a while now.
As others have said, a free proxy is not a good alternative, they are harvesting just as much data as your ISP, if not more (and possibly selling the info back to your ISP). Free proxies also have little incentive to be honest so there is a higher chance they could also be manipulating your traffic in various ways.
A better solution for mitigating data harvesting from your ISP is to just simply adjust your personal DNS settings. This can be configured globally on your router sometimes and/or each client device. Change your default DNS to a more privacy conscious provider such as Mullad, Quad9 or even Cloudflare. Those DNS providers also offer the option of making encrypted DNS requests using DNS over TLS (DoT) and DNS over HTTPS (DoH) which can be configured in your browser and sometimes your router.
At the end of the day, your ISP will still know you connected to a specific end point but they can't determine what exactly you are doing there, they only know you connected to something like Lemmy.world but would not be able to determine which communities you are visiting.
If a proxy still feels like the only solution then just buy a VPN subscription from a reputable source, its usually pretty cheap and way more effective at preserving privacy and security than any free proxy that is available. Mullvad is generally a pretty good and inexpensive choice plus they also have pretty decent proxy support.
As an alternative to FoxyProxy, check out the extension Container Proxy which allows you to containerize a tab and route all the traffic through a configured proxy which can also be configured on a site-by-site basis. Still, not perfect, but certainly a step above FoxyProxy.