Only telemetry, which is not that harmful to user, actually it helps making Firefox better, which users want, right? Or probably not, as we all turn it off. The biggest data spy tool is Firefox Account, but you loose sync than. LibreWolf turn it off by default and if you don’t use sync it is way bigger step to privacy, than turning telemetry off. Also there is a way to setup own sync server, other browsers do not allow it (but you are not forced to use accounts anyway). Anyway that is probably not that bad, if you are not paranoid lol, I even use Firefox Nightly, not Mull, for newer features, etc.
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You can turn off data collection in Firefox
Only telemetry, which is not that harmful to user, actually it helps making Firefox better, which users want, right? Or probably not, as we all turn it off. The biggest data spy tool is Firefox Account, but you loose sync than. LibreWolf turn it off by default and if you don’t use sync it is way bigger step to privacy, than turning telemetry off. Also there is a way to setup own sync server, other browsers do not allow it (but you are not forced to use accounts anyway). Anyway that is probably not that bad, if you are not paranoid lol, I even use Firefox Nightly, not Mull, for newer features, etc.