I have recently reinstalled Firefox (my OS is macOS 10.14 Mojave) and the Telegram web client no longer remembers my login. Always used to work fine before, but not anymore.
Every time I start Firefox and visit
web.telegram.org I have to sign in again through phone number + sms.
In Firefox' Privacy and Security settings, I already whitelisted
web.telegram.org to store and keep cookies + localstorage, and in fact when I look in Manage Data it does show several MB for web.telegram.org being stored.
But nonetheless it keeps forcing me to re-login every time. Also if I completely delete all site data and history and cookies and everything, and try again, after logging in, closing and restarting Firefox, I have to login again.
The same works fine in other browsers (tried Chrome and Brave, both OK) but I prefer to use Firefox.