Maybe a silly question - but is it unwise to use Firefox for getting torrents, or saving any bookmarks in firefox? Is there benefit to using a private window (doubtful as I believe this only affects your device).
I know we generally can trust Firefox but they could turn quickly.
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.