I used it for a bit during its early years mostly for RSS feeds and newsgroups.
I suppose you want to aggregate your email accounts into TB? On my phone, I have Gmail (family/friends and spam stuff), Outlook (work) and Proton Mail (government/taxes, banking/investments), and I like to keep those accounts separated. On desktop, I simply use my browser. I personally don't have a use for a mail client today, but if it's something you find attractive then, sure, TB is great if you have privacy in mind (and we know Mozilla has that covered).