Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

No, it's not unwise. Mozilla has no mechanism with which to surveil your activities built into the browser.

That said, you should avoid categorising companies as generally trustworthy or untrustworthy. Any given service will have privacy considerations - some may be important to you, others may not.

If you do not sync your bookmarks and such, and you do not install malicious plugins, then this information is as safe as your device itself is.

Firefox claims to use E2EE so It should be pretty safe to use their built-in sync as well.

It's open source so if you feel like verifying all this, or compiling it on your own, you can.

Or you can use a completely separate instance or browser (perhaps ice weasel or even the Tor browser) just for the activity you want to keep separate.

I recommend Librewolf, it's a lot more privacy-aggressive out of the box, and you can turn that down a little bit if you need, but otherwise it's just a more trustworthy Firefox fork as far as I'm concerned. It supports Firefox sync as well (which is telling, because Librewolf takes privacy very seriously and isn't going to provide too many easy opportunities for you to completely compromise it) Like the other person said sync is E2EE and the hosting server has zero-knowledge of any of your unencrypted data. If Librewolf trusts it, I trust it, and I think you can rest assured that with Librewolf, it's probably never going to be sabotaged either, which as you imply, is not necessarily true with Firefox.

I don't recall whether they use Firefox's sync server directly or if they have their own, but either way, like I said, the server has no knowledge of or access to your unencrypted data.

Browsing or even downloading torrent files, or keeping bookmarks, is not illegal. So, it doesn't matter.

The private window prevents your local from storing your browsing history, which in turn means it doesn’t sync with your Firefox account.

The only danger when torrenting/pirating is the download of the actual content itself. With torrenting, a copyright holder will use the torrent to find connected peers and send CnD's to the IP issuers. With regular downloading, the site you're downloading from can see your IP and this comes with the same risk of an CnD if the site is bugged/infected. Other than that, browsing torrent sites or download sites themselves carry no risk outside of maybe malicious advertisements or popups.

private windows won't add your browsing to your history. there is no benefit to using private windows otherwise. it's sole purpose is to hide your visits to sites you're embarrassed about from others using your PC.

Firefox takes like 3 minutes to launch when my VPN is connected so I started using libre wolf