Personally I am not aware of any such breaches, but even though I really like Mozilla I still try to obfuscate data collected about me by any entity.
Your data is not a commodity you should be handing out freely if at all avoidable.
Monitor is have I been pwned with extra steps, omnivore is an open source alternate to pocket, vpns are everywhere, there are other file sharing apps like relay
Relay isn't file sharing though. It's an email and phone number masking service. I signed up and played with it a bit. Didn't see much use in it til recently and I signed up for the premium. Figure I waste more than that a year on stupid things and this way somewhat supports the browser I enjoy using. 🤷♂️
Pocket is good for saving articles for offline reading. While I rarely need it, when I need it, it's super helpful. Also it can be used as a shared bookmarks storage. I don't share bookmarks because I don't want to mix personal bookmarks and bookmarks from work, so Pocket is useful for me.
Relay is a cool thing for one-time emails, but I rarely need it.
I use Pocket as temporary bookmarks, for e.g. crafting lists in games, or which page of a comic I'm on, or timestamp within youtube videos. It's so quick to create, rename and delete entries in the addon, no need to think about tagging and proper organisation like with my long-term bookmarks.
I also use Thunderbird as an offline storage of my Gmail emails, but I don't actually use it as an email client.