Constant logging out

Did you delete browsing data or you turn on the delete browsing data on quit? that might the problem

No, I didn't touch any settings appart from the update itself. No other website is acting like that also.

Same here. Started happening after Firefox auto-updated to version 154.0 yesterday. I gave it some time, thinking it would get fixed quickly, but am now starting to get annoyed (& a bit concerned). I use a MacBook Pro, running OS 15.7.7 Sequoia. The browser asks me to log in EACH & EVERY TIME I start it up. My password manager is also EMPTY. Luckily, my passwords are present in the Firefox app on my iPhone. I ran "Sync" a couple of times from the app, hoping that might help, but there are still ZERO passwords saved in version 154.0. I'm shocked we're the only people commenting on this -- maybe there's a related thread somewhere else (??).

This has been my experience with 154.0 too. Running macOS 26.5.2 on an iMac. The Firefox update preserved my bookmarks and browsing history, but I can't access any saved passwords. And the login only lasts as long as I leave the browser open. Could these problems be related to the fact that there are now two profiles sitting in my Firefox library folder ? Default and default-release. Only the second one contains a logins.json file that predates the move to 154.0.

It was a cookies issue. Following advice from Mozilla support I changed the enhanced tracking protection settings from ‘custom' to ‘strict'. This resolved the constant signing out.