"Keep me signed in" stopped working

Jeremiah

I never used to have a problem with this, but recently I keep getting logged out whenever I restart my PC or browser.

Using Firefox on Windows 10, always logging in from the same connection, and I've ticked the "keep me signed in" box.

Edit: did some experimentation to see if I could find anything that might be causing it.
It only seems to happen in Firefox, I tried Edge and Chrome and it was OK
It doesn't happen consistently; I thought it might be related to the number of browser tabs I had open when I hit "restore previous session", it seems more likely to log me out if I open more tabs, but that's not always the case.