I have had the same problem for quite a long time, I get logged out after 30 minutes or so on this and some other sites as well, using Firefox and Pale Moon and Chromium browsers, all seem to be affected, all being used on a desktop WIN 8.1 platform. I suspect it has to do with privacy settings rather than any specific piece of software. I run a number of security enhancements on the browsers, so I just deal with it as necessary. I just have to log in (like now) to post or reply. Not a huge deal, I can live with it.