Session Expiry issue

I am getting a session expiry message with a demand to login again, after only a few minutes doing posts to blogs, sometimes because I open another tab in my browser (and possibly another blog) – but even if I check an email, do research, or look for something on my computer. I never leave the post for more than a couple of minutes.
This has never happened before. I’m using the Firefox browser, most recent update.
Is there a setting to adjust this? It’s extremely annoying, since I lose any recent edits to the post I’m working on.
A search didn’t turn up anything for this.