If Firefox is deleting cookies and site data when it closes, you will be logged out of any site. This will also happen if any third-party cleaning software is set to delete cookies.

In the Firefox Preferences, ensure cookies and site data are not set to delete when Firefox closes.

You can use this URI in the address bar - about:preferences#privacy or navigate from the Firefox menu and select the Preferences entry, and select the Privacy & Security page from the column on the left.

1) In the Cookies and Site Data section, ensure the tick box for Delete cookies and site data when Firefox is closed is empty.

2) In the History section further down the page, if Use custom settings for history is selected, and Clear history when Firefox closes is also selected, then ensure that the settings for this do not include Cookies, Active Logins, and Site Data.

Sorry, I checked both settings, and they were correct. Well, I have FF set to remember history, so the second setting was not necessary. I also looked at the Check Data page, but I could not understand it, so I left it alone. By the way, yes, I have the Keep me signed in button checked on the whatsapp.web page!

I have been looking into this on and off all day. This used to be a generically bigger problem, but i don't see much recently, especially since whatsapp was allowed to connect to more devices, and device support got better. I don't see much of anything in the whatsapp web documentation, either.

When it disconnects, do you receive an error message such as "Phone not connected" , "Computer not connected", or is it simply not connected any longer without mention?

McCoy, another volunteer here, suggested perhaps if you have Firefox remember your session, this would help. In your Menu → Options, there is a checkbox for Restore Previous Session right at the top in the Startup section.

If nothing else is helping, maybe installing and using the whatsapp desktop app and escape the frustrating issue for now: https://www.whatsapp.com/download/

Thank you for your hard w3ork! No, the web.whatsapp page simply reverts to the sign-in screen, as if I had not just been connected. Curiously, on my Iphone, the Whatsapp app shows that there is still an active session, although it identifies it as Windows, and not Firefox. Yes, I tried reopening FF with the previous session recovered, but that did not help. As for the Whatsapp desktop app, sigh, like so many other such apps, it does not play nice with the TTS software. Again, I do not want to be a whiner. I have the workaround on FF, clunky as it is, and I can also just open Edge, connect to web.whatsapp, and use that for my messagin while otherwise browsing with FF.

Did you try to create a cookie allow exception for the Whatsapp website ?

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Permissions"

What a great idea! Sadly, I just tried it, and no, it made no difference. Thanks anyway.

I hope this works better in the future. Best wishes.